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Immanuel Kant


‘Thoughts without intuitions are empty, intuitions without concepts are blind.’

                            Immanuel Kant – ‘Critique of Pure Reason’, B 75

Or . . . Reason (Rationalism) cannot work without the percepts of our sense experience (our perception): experience (Empiricism, science) cannot work without the concepts of our mental structuring processes (our cognition).

Science works, so there seems little point in wasting time agonizing about its underlying assumptions, foundational principles, and the mental preconditions that make it possible. Lets just get on with the experiments.

And yet . . . I hope that after reading this article you will agree that Immanuel Kant gives us some valuable insights into the way that we interact with the world in general, and about the assumptions that lie behind science in particular . . . he helps us to think critically about many of the intuitions we all have about the way things are. He is a touchstone for thinking about the way our mental faculty works with sensory information to provide us with meaningful experience.

Many intellectuals consider Kant to be the greatest philosopher since Plato and Aristotle. He was from a time when philosophy and science were one and the same, and he provides a critical link between the philosophical problems of the ancients and those of the modern era. For this reason alone it is worth taking the time to consider what he was trying to achieve in his work. He also exhibits in his personal life the contradiction that so often appears in human affairs. Though widely regarded as one of the world’s greatest ever moral philosophers, he was racist in extreme. This might counterbalance any inclination to philosophical hero-worship we might want to indulge . . . and is a substantial philosophical puzzle in itself.

This brief introduction to Kant outlines some of the key ideas that have occupied philosophers throughout human history. As in other articles on the site, I have stated key claims as ‘principles’ for you to seriously consider and challenge.

Immanuel Kant

Portrait c. 1790 – painter anonymous – 1724-1804

Courtesy Wikimedia Commons – Amano 1 – Accessed 8 September 2016


Immanuel Kant (1724-1804) was a Prussian academic and pietist, a popular lecturer at the University of Königsberg (today’s Kaliningrad in Russia) near the Baltic coast. Biographic accounts mention how the citizens would set their clocks by the obsessive regularity of his daily constitutional as he passed their houses accompanied by his manservant. For 40 years, in spite of his lack of travel, he gave extremely popular summer lectures on physical geography. His Physische Geographie was delivered 48 times, and his insistence on the unity of knowledge, whereby facts interlock with others within a web-like framework, undoubtedly influenced fellow Germans Goethe and von Humboldt (who displayed a bust of Kant in his study)[10] and reappear in the philosophy of 20th century American Willard Van Orman Quine. He lived at the height of the European Enlightenment[3] when the progressive autocrat King Frederick the Great ruled Prussia from 1740 until 1786. In 1770 he was appointed Professor of Logic and Metaphysics at the university.

Kant had a deep intellectual respect for the scientific achievements of Isaac Newton. Like many other philosophers of his day, Kant was acquainted with the full range of human knowledge, not just philosophical puzzles. In 1755 he had argued that gaseous clouds (nebulae) slowly rotate, gradually collapsing and flattening due to gravity, to eventually form stars and planets (the Kant-Laplace Nebular Hypothesis) and that the universe was expanding. At that time six planets were known: Earth, Mercury, Venus, Mars, Jupiter, and Saturn. Kant predicted that more planets probably lay beyond Saturn, then in addition, realizing that our solar system was just part of the more extensive Milky Way Galaxy he proposed that other planets must exist outside our solar system. The first prediction came true in Kant’s lifetime when Uranus was discovered by English astronomer William Herschel in 1781, the year of publication of Kant’s most accomplished work the Critique of Pure Reason (the first of three critiques) and the same year as the signing of the American Articles of Confederation. Confirmation of his second prediction would take longer. He also pioneered anthropology, lecturing in the subject for more than 25 years and publishing Anthropology from a Pragmatic Point of View (1798), only printed in German in 1997, and English in 2006.

Politically, Kant believed that world peace could be secured through universal democracy and international cooperation and it was he who, in his moral philosophy, propounded the highy influential moral theory known as deontology with its now famous categorical imperative (‘Act only according to that maxim whereby you can, at the same time, will that it should become a universal law‘). But of more direct interest to us here is the side of his work that touches on aspects of human nature and the philosophy of science, including its implications for cognitive science and evolutionary biology.

Kant’s broad philosophical program was an attempt to deal with the philosophical problems of his day. His achievement was to advance human knowledge of, among other things, space, time, substance, and cause – together with a closely-argued account of the bounds of reason, understanding, perception, cognition, experience, imagination, knowledge, science, metaphysics, morality, religion, and reality.

Knowledge – its acquisition and limits

Knowledge is justified true belief

Knowledge is experience with understanding

Knowledge is not built up from secure premises like a foundational deductive system; it is more like a coherent web of beliefs that works
Coherentism. Pragmatism. (see science & reason)

From its beginnings in antiquity, philosophy was concerned with the question of what we can know, and how we can know it (epistemology), on the distinction between appearance and reality or, in other words, the quest for certain knowledge. Though a seemingly obscure matter, both our intuitive and carefully considered conclusions about such matters can affect our attitude to entire domains of our experience . . . like science, mathematics, logic, morality, and religion.

In the modern era philosophy took an ‘epistemological turn’ by concentrating on the constraints to knowledge imposed by our minds (philosophy in the 20th century would take a similar ‘linguistic turn’).

Four general theories of knowledge had been inherited from antiquity, with variants of these persisting to the present day: empiricism – that all knowledge is ultimately derived from sense experience from which we then make inferences and generalizations (?Aristotle, Locke, Berkeley, Hume); rationalism – that certain knowledge can only come from within us as a product of reason (Plato). One variant of empiricism was rational empiricism – the view that the mind applies to our experience the organizing laws and categories of thought (the sensus communis) which correlate directly with laws and categories of the external world (Aristotle); and another variant, skepticism – that knowledge, at least certain knowledge, is impossible (Pyrrho, and Sophists like Gorgias, Hume).

From at least the time of Socrates philosophers had given pride of place to reason as the source of knowledge, as the unique way that humans uncover the necessary and universal truths of mathematics, logic, and the world (metaphysics).

Plato maintained that the knowledge gained from experience and observation, which included scientific knowledge, though valuable, was imperfect and inferior because we are easily deceived by the senses: such knowledge could not possibly challenge the universal and timeless truth so beautifully demonstrated by mathematics.

Aristotle also had a deep respect for reason, in fact he regarded it as the single defining human characteristic, setting humans above all other creatures. He graded the living world into organisms that displayed nutrition (like plants) to those that were, in addition, motile (animals), to creatures that were aware (sentient animals), to those that were not only aware but had the capacity to reason (humans). Even so, Aristotle believed that ultimately all knowledge must have its origin in the senses (‘There is nothing in the intellect that was not previously in the senses‘) he was not a Rationalist like Plato, he was an Empiricist.

The Platonic and Aristotelian views about how we obtain knowledge – their epistemologies – were taken up by later philosophers. In the mid 18th century philosophy arrived at an impasse, a deadlock between the Rationalists and Empiricists who appeared irredeemably opposed to one-another. On the one hand there was the continuing line of Rationalists that included (after Plato), Descartes, Spinoza, and Liebniz, all conviced that knowledge derives from our use of reason. French philosopher Descartes, for example, was famous for his dictum cogito ergo sum (I think therefore I am). For Descartes the act of thinking and a sense of self or ‘I’, along with intuitive morality and the idea of God, were all forms of knowledge that were prior to (a priori) experience as sensory input. Empiricists were not convinced by this and they found a strong voice in 18th century Britain.

The German Kant characterized this debate as a disagreement between dogmatists (rationalists) and skeptics (empiricists) and he is celebrated as the philosopher who was able to unlock this dilemma by demonstrating that the distinction between Rationalism and Empiricism was a false dualism – both were needed for the acquisition of knowledge. Kant’s thinking is still highly relevant today.

Rationalism vs Empiricism

The Enlightenment had stimulated a new enthusiasm for science and reason and Kant was determined to subject the foundations of these two intellectual tools to the closest possible scrutiny.

In Britain there had been an upsurge of empiricism promoted by a trio of great thinkers: Englishman John Locke (1632–1704), Anglo-Irish Bishop George Berkeley (1685–1753), and Scotsman David Hume (1711–1776). Hume, especially, had mounted a compelling argument against Rationalists arguing that the necessary truths of mathematics and logic were merely analytically true, that is, they simply followed an internally consistent relation of ideas like the axiomatic system of Euclidian geometry, but ultimately they conveyed only trivial tautological truths: they had nothing to say about the world itself – about matters of fact – only empirical science could do that. Hume pointed out that logic is a method, it is not a mode of discovery.

The distinction between ‘matters of fact’ and ‘relations of ideas’ became known philosophically as ‘Hume’s Fork’. Further, Hume maintained, even matters of fact (like the laws of nature) are not necessarily and universally true, they are only contingently true since they depend on the ‘uniformity of nature’ (as he called it). Further, Hume maintained that there was nothing in experience that justified causation as commonly understood. That is, causation was a creation of the mind, not a matter of fact. So, for Hume, science rested on a false belief in cause and effect and an unjustified faith in the uniformity of nature. And, since the grand metaphysical systems of philosophers about the nature of the world were neither empirical nor analytic (true in virtue of their word meaning) they were, at best, incapable of intellectual resolution and, at worst, meaningless.

In short, Hume’s skepticism claimed that there could be no knowledge independent of fallible experience. He summed up his philosophical position as follows ‘If we take in our hand any volume; of divinity or school metaphysics, for instance; let us ask, Does it contain any abstract reasoning concerning quantity or number? No. Does it contain any experimental reasoning concerning matter of fact and existence? No. Commit it then to the flames: for it can contain nothing but sophistry and illusion.’

While science was clearly making constant, even accelerating, progress, metaphysics seemed mired in controversy. Hume had joined the chorus of voices condemning metaphysics as endless speculation and conjecture.

Hume’s attack on rationalism shook Kant from his complacent conviction that both science and religion were intellectually secure – that physics, like mathematics, is a body of necessary and universal truth, and that reason plays a foundational role in the acquisition of knowledge. The Empiricists pushed Kant into his famous rebuttal of Empiricism The Critique of Pure Reason (1781)[1][2] a project that took him 11 years to complete.

Though highly influential, the insights of the Critique are notoriously difficult to penetrate in both the original German and the English translation not only for their depth and subtlety but because of Kant’s prolixity – his mind-numbing use of technical jargon. He approached his subject as a system-builder with the great skill of integrating many complex ideas into a coherent thesis. None of the depth and range of his thought can be conveyed here but a very brief outline of the ideas expressed in the Critique will convey the flavour of Kant’s thought and its significance for scientists and scientific thinking.

Kant called his famous counter-argument to the empiricists transcendental idealism although, after being accused of idealism, he preferred the name critical philosophy. He was using ‘transcendental’ in a special epistemological way to indicate ‘that which makes experience possible‘, not in the usual ontological sense of ‘transcendant’, meaning something that exists outside and beyond this world and therefore unknowable. In the title of his work Kant used the words ‘critique’, meaning ‘critical analysis’ rather than ‘criticism’, and ‘pure reason’ to mean reason stripped of all experience – reason, and therefore potential knowledge, that arose within the mind, uninfluenced by external factors.

For Kant our experience of the world must entail a linkage between what is perceived and what is thought. A good place to start is therefore with his account of perception (the way we access the external world with our senses) and cognition (the mental processing of our perceptions, ideas, images, emotions, and other mental representations).

Phenomena & noumena

Kant called physical objects in the world noumena and he noted that we know these objects as appearances or representations in our minds, which he called phenomena.[8] We thus have two kinds of things: first, suprasensible mind-independent objects in the world and, second, the representations of these objects in our minds. Kant called an object in the world the ‘thing in itself’ (das Ding an sich).

Contemporary commentary

This apparently simple characterization of our experience needs some contemporary commentary in the light of post-Kantian evolutionary theory and the many philosophical confusions and interpretations that have subsequently ensued:[6]

1. We experience the world through the integration of our human perception and cognition, with all the constraints and limitations that this entails

2. This is a species-specific, human mode of experience that arose out of our unique evolutionary history as a species. We know that other sentient organisms experience the world differently

3. This does not mean that our experience of the world is mistaken or illusory – but it is a partial and human mode of experience

4. We obtain a more complete experience of the world by extending the range of our senses using science and technology – microscopes, telescopes, computers, chemical analysis and so on. To our findings we apply the exceptional computing power of our brains

5. We can imagine the possibility of a ‘complete’ view of the world but realize that this would entail the impossibility of experiencing the world from every conceivable perspective. If this is what is meant by das ding an sich then we must concede that such a perspective is impossible and das ding an sich knowable only through our biologically determined experience as extended by science and technology

A major point here is that we can always imagine more complete knowledge of objects than we actually have (there is no end-point to science) and that the idea of having a complete knowledge of the external world is incoherent.

This point is a source of much confusion and, not surprisingly, Kant was not completely clear in expressing his own views. This complication will be discussed further in Objects.

Principle 1 – Reality fictionalism. Objects exist independently of observers. However, there can only ever be perspectives or interpretations of these objects as defined by the limitations of the perceptions and cognitions of the observer. These perspectives are not illusory or mistaken but they are partial. The expression ‘thing-in-itself’ implies a privileged perspective that is unattainable – a God’s-eye view, or a view from the point of view of the universe. The ‘true nature’ of the ‘thing-in-itself’ is a kind of ideal perspectiveless knowledge which is the unattainable goal to which science aspires (discussed later). As Liebniz expressed it, reason attempts to describe the world from ‘no point of view

The ‘thing in itself’ is a form of philosophical fictionalism: it has no intelligible referent and yet we need the concept as a placemarker for any object of interest that can be interpreted in different ways – so we use it as a limiting concept (fictionalism may also be applicable to key ideas like ‘truth’, ‘reality’, ‘number’, and ‘morality’): without it we would have only phenomena in our heads when we ‘know’ that the world does, in fact, contain mind-independent objects.

The actual way we experience the world directly (our ‘reality’) is determined by the evolved biological constraints and limitations of our sense organs. In nature there is no single ideal organism towards which evolution has been directed (though some would put humans in this category), only bodies adapted to particular environments: so there is no ideal form of cognition that gives a unique and true experience of the world, of reality . . . there are only forms of cognition that are adapted to particular modes of life. But just because we operate with only partial knowledge of the world (one of many interpretations), does not mean that the idea can be ignored (eliminativism) because it is needed as a placemarker. These points need to be kept in mind whenever noumena or ‘things in themselves’ are mentioned.

Perception – sensing

We can say with confidence that each kind of sentient animal experiences the world in its own particular way depending on the capacities of its biological sensory apparatus. Humans perceive the world differently from birds, dogs, worms, and bats. Moreover, in spite of the vast processing power of the human brain, human perception does not have a privileged access to reality: our perception is limited in the same way as the perception of any other animal. For example, we do not have the olfactory or auditory ability of dogs or the visual acuity of birds; I cannot see in the ultraviolet range of light as bees can, or use echolocation for night ‘vision’ like bats. So in these ways, and many others, our perception of the world is limited or partial. All us humans can experience of the world, in an immediate sense, is what our biology allows us to experience.

Cognition – thinking, reason, and understanding

All our experience is experience of something, of objects of some kind. Conscious experience is not a chaos of visual, auditory, and other sensations pouring into our minds in a disorganized blast of mental stimulation, what Plato had called the ‘rabble of the senses’. Our minds act like a filter, converting all these sensations into coherent mental representations, what Hume colourfully described as the ‘theatre of the mind’. So, for example, though our visual perception of a person walking towards us is constantly changing, we discriminate the ‘person’ from the person’s surroundings, and our concept ‘person’ remains stable while the visual perception changes. Seeing a tree is different from just seeing: seeing a tree assumes a particular object placed in the general conceptual category ‘tree’. So:

Principle 2 – the objects of perception (percepts), along with many other mental objects, are organized by our cognition into mental representations (concepts) such that the world makes sense. Together, percepts and concepts make up our world of experience and understanding.

Another way we refer to this mental structuring is to call it ‘reason’ and Kant demonstrates how we can see the operation of reason as both an unconscious automatic or innate process and a conscious act of deliberation.

Principle 3 – Reality structuring. From a modern perspective we can distinguish two kinds of mental structuring: on the one hand there is the intuitive or innate structuring that is part of our inherited biology, and then there is conscious reasoning. It is the unconscious structuring process hard-wired into our biology that makes experience meaningful. Kant had a magnificent expression for this automatic mental structuring into meaningful understanding – he called it ‘the synthetic unity of our apperception‘.

Cognitive spectacles

To help understand Kant’s thesis and its significance imagine viewing the world through spectacles with blue lenses. The world itself (whatever that is!) is not blue, but to us it looks blue because the lenses are acting like a blue filter, making everything appear blue. In this way our uniquely human mental predispositions act like a mental filter between us and the world. This does not mean that we are in error or that the world is an illusion, just that we experience the world in a particular human way. This form of knowledge is not like factual knowledge but more like a grid or framework that structures our perception and cognition.

Philosophical Copernican Revolution

One of Kant’s great insights, sometimes referred to as his Copernican Revolution in philosophy, was like the move in physics from geocentrism to heliocentrism. Just as in physics the Sun only ‘appears’ to orbit the Earth, so the world we experience only ‘appears’ to us from our human point of view. This was in stark contrast to the prevailing Enlightenment perspective which presumed the objectivity of all perception and knowledge – sometimes called the ‘spectator’ view of reality – in which humans are just observers, not participators. He made us aware that the mind is not a passive interpreter of the world, it is not a ‘blank slate’ (the tabula rasa of empiricist John Locke) on which experience can write its story – the mind plays an active role in structuring our experience with the a priori concepts of our precognition.

Most importantly, as discussed above, Kant emphasised that we cannot take our spectacles off and see the world as it ‘really is’. Seeing the world as it actually is does not make sense because all organisms experience the world through their own particular cognition, their inherited biological spectacles, and there is no vantage point from which to view the world in an independent ‘objective’ way (but see Reality and representation later). Kant referred to this filtering of knowledge as transcendental because it existed independently of our experience of the external world (though acting on that experience). The claim that all we can know of objects in the world (noumena) are our mental representations of them (phenomena) Kant, and others, referred to philosophically as ‘idealism’. For these reasons Kant called his philosophical position transcendental idealism. Through his transcendental idealism Kant claimed to have shown that Rationalism and Empiricism were mutually interdependent and that is what he meant by (paraphrasing) ‘concepts without percepts are empty and percepts without concepts are blind‘. That is, empirical experience (empiricism) must be mentally processed (rationalism) to produce meaningful knowledge (understanding) of the world.

According to Kant the process of conscious deliberation we call reason proceeds according to the innate structuring processes of the mind. These structuring processes, essentially the rules of thought, Kant called the Categories (considered later).

This brief outline is a modernized take on Kant’s general ideas. To follow his philosophical journey in more detail we need to come to grips with some technical terms.

The limits to knowledge

Four kinds of judgement

Kant tried to crystallize the distinction between Empiricism and Rationalism by distinguishing opposing pairs of judgements that summarized the possible ways in which we can gain knowledge.

First there was the question of the source of knowledge, whether a priori or a posteriori:

A priori = known by reason alone, independently of experience (e.g. maths – once the word meaning is established). This is a form of knowledge that is both necessary (could not be other) and universal (true without exception)
A posteriori = only known as a consequence of experience e.g. science

This distinction is also sometimes expressed as the contrast between the necessary – as true in all possible worlds and the contingent – as not necessarily true in all possible worlds.

Then there was the kind of knowledge that we obtain from word meanings and logical inference. This is of two kinds, analytic or synthetic:

Analytic = determined by word meaning alone (true by definition) – ‘All bachelors are unmarried men’ (analytic – accessed by pulling apart). For Kant (but not later) the predicate concept is contained in the subject concept: the conclusion is in the subject and therefore adds nothing.
Synthetic = determined by both word meaning and the world – ‘All bachelors are happy men’ (synthetic – accessed by putting together). Kant characterized this as the predicate adding to the subject by being, as Kant puts it, ‘ampliative rather than explicative’ that is, genuinely informative.

Hume had made just one epistemological distinction:

Analytic/a priori/based on reason = necessary & universal = ‘relations of ideas’
Synthetic/a posteriori/based on experience = contingent = ‘matters of fact’

This was Humes Fork (already mentioned) and his conclusion was that all knowledge consists of either informative but contingent ‘matters of fact‘ based on sensory experience (empiricism), or necessary and universal knowledge that is uninformative and trivial ‘relations of ideas‘ which were a consequence of reason (rationalism).

Hume had summarily dismissed rationalism and asserted that only experience can provide knowledge, and only contingent kowledge at that – it could never be necessary or universal.

This deeply shocked Kant who took ten years to respond with his Critique … which insisted that for a full account of knowledge all the possible combinations of the different kinds of judgements listed above should be considered:

Analytic a priori – (true by word meaning and known prior to experience) logical truths, for example a 3-sided triangle (once initially explained)

Analytic a posteriori – (true by word meaning but requiring experience): incoherent since there is no need to appeal to experience

Synthetic a posteriori – (requiring experience) for example a red triangle: uncontroversial matters of fact

Synthetic a priori – (something about the world that is known without experience). New knowledge that is necessarily true, a category denied by the empiricists

The impasse

Hume regarded synthetic a priori knowledge as impossible (if it is empirical it cannot be necessary and universal) and it is what Kant has to say about this that has challenged philosophers and scientists to the present day. Kant argued that reason and experience were not mutually exclusive forms of knowledge because some a priori judgements are synthetic. In the Critique . . . he sets out, in fine detail, to explain his grounds for these synthetic a priori judgements. Though all knowledge begins in experience not all knowledge arises out of it. His work was an extended philosophical answer to the question ‘How is synthetic a priori knowledge possible’. His answer, to use our language, is that synthetic a priori knowledge is the innate structuring imposed by our perception and cognition, the knowledge that we derive from our human cognitive ‘spectacles’. Kant pointed out that the empirisist claim that synthetic a priori knowledge was not possible was itself a synthetic a priori judgement.

Synthetic a priori knowledge

Empiricists pointed out that we are not born with knowledge of the world: that is a ridiculous claim. Kant, in a direct challenge to empiricist epistemology, argued that in an unexpected way this is precisely what does happen. Though knowledge arises in experience, this experience must be structured by our cognition if it is to make sense. Cognitive structuring, though not a familiar form of knowledge, comes from within us – we need it to survive. It is not the usual kind of factual knowledge of the world but without it we could have no meaningful experience – it is knowledge arising from the innate predispositions of the mind.

This Synthetic a priori knowledge (the non-experiential inner structuring of experience) was neither true by definition (analytic) nor a consequence of experience (a posteriori). This was Kant’s challenge to empiricist epistemology.

More importantly, Kant insisted that synthetic a priori judgements underpinned the way we predict and explain the world, including the propositions and judgements of mathematics, science, and metaphysics. Because these innate characteristics are, as it were, compulsory (we cannot remove our innate filters of perception and cognition) they constitute necessary and universal knowledge – they are a mental structuring tool or lens that provides us with a mental framework or grid that gives us a meaningful understanding of the world.

This is what Kant referred to as ‘transcendental’ (before experience) knowledge.

Kant’s transcendental idealism closely resembled Aristotle’s rational empiricism. Kant, however, did not share Aristotle’s confidence that our minds accurately mirror the world. Kant understood that we have a human-eye view of things – that there is biological mediation. Aristotle, it seems, did not regard our immediate view of the world as incomplete or an ‘interpretation’ – a view still supported by many today.

Predispositions of the mind

But how can we possibly know anything about the Categories, the predispositions of the mind (the innate cognitive structuring) that Kant speaks about, aren’t these enabling processes pre-conscious?

The Transcendental Method

Kant called his answer to this question the transcendental method. Though seemingly an impossible and highly controversial project Kant maintained that it could be done by stripping away all the content of our awareness that has its origin in experience and discovering what was left. He asked ‘What are the preconditions that make perception and cognition, our interpretive understanding, possible?‘ This was a refreshingly different way of asking about how we acquire knowledge, a radical departure from the philosophical skepticism of someone like Descartes. Kant took it for granted that we do acquire knowledge: the important question for him was ‘How is this possible?‘.

Kant maintained that to make objective judgements about the world we need two necessary and universal principles (what he called Transcendental Principles), the predispositions of sense perception (which he called the Transcendental Aesthetic) and the predispositions of cognition or understanding (which he called the Transcendental Analytic). He then set about describing these predispositions as ‘Categories of the understanding’.

Kant, in his philosophizing, appeared to be doing what we might call science: but he was not performing an empirical investigation, only exploring what must be logically presupposed in order for science to occur at all . . . the minimal requirement for meaningful experience to take place.

A priori modes of sensing: categories of perception

Kant began by noting that in order to have any mental representations at all we must presuppose space and time as perceptual Categories. Space and time are the form of all perception: they are what make possible our experience of everything. For Kant, space and time are not properties of objects in the external world but the preconditions for perception itself. They are therefore a priori and necessary.

The Transcendental Aesthetic

For Kant space was ‘the pure intuition of outer sense‘ and time the ‘pure intuition of our inner sense‘ with intuition referring to our basic awareness. In simpler and more modern terms, space and time are not ‘containers’ given to our experience from an external world but internal filters determined by our biology. Time for Kant was also ‘the form of the reflective consciousness, common to both sense perception and thought, to all the pure Categories‘. We cannot think otherwise: thought is ‘in’ and ‘of’ space and time.

Kant’s conclusion strikes most of us as being both confronting and unlikely. He is saying that space and time are neither empirical data, nor concepts, they are the forms by which we experience the world. Since we cannot imagine space and time independent from experience (they are not learned) they lie beyond experience, and as necessary conditions for experience they are ‘givens’, they do not require proof. Put simply, space and time are a priori intuitions.

So for Kant, though we can have concepts (empirically-based evidence) of space and time there must also be underlying inner intuitions to make this possible – an outer sense of space, and an inner sense of time.

The Categories, when operating together, permit understanding and the making of empirical judgements. For Kant they are ‘pure’ concepts; they are not (as Hume had claimed) derived from experience, but have their origin in the constitution of the mind itself. Today, we would say that they are innate faculties of our cognition. It is because the judgement is so grounded a priori, that we are entitled to assert the principle of causality: that all events of type A are universally and necessarily followed by events of type B. This principle is ‘transcendental’ in this strictly technical sense: it is known to be true not from experience, but because it is a condition that must be fulfilled for empirical knowledge to be possible.

Kant provided six arguments in defence of space and time being a priori. The main drift of these being the question ‘How we can get an impression of space from the external world if we need the notion of space to experience the world at all?’ Perhaps the most persuasive argument comes from our incapacity to imagine their absence. Since our minds automatically place everything within the context of space and time it is no surprize that we have the greatest difficulty in taking ourselves out of this innate biological mind-set to think of a beginning or end to time, or a boundary to space. Time simply cannot end and the boundary to space, we intuit, must have space beyond it. This is a consequence of the biological limits to our cognition.If our sense of space is created by objects that are separate from us, this separation presumes the existence of space. Kant supported his thesis by pointing out that though we can think of empty space as an absence of objects, we cannot think of an absence of space itself: we cannot represent to ourselves an outer world empty of space. Also we can only represent one space: and all other spaces are in that one space. This is not true of other (non-intuited) objects like chairs and tables which are limiting concepts not intuitions. Space is infinitely large and infinitely divisible in a part-whole relation while objects like chairs can have an infinite number of representations under itself but not within itself (like space). Another argument he suggested to illustrate his point was to ask ‘How do we tell the difference between two identical gloves set out as right and left?’ The difference is not made intelligible by any concept only by a difference in space (intuition) so we can only point.

The contrast between space and time as phenomena and the world of spacetime presented to us by physics (the closest we can get to the noumena) seems to vindicate Kant’s assertion that since we cannot experience the world directly only as our human interpretation.

Kant was aware of the famous dispute between Liebniz (an early influence on his thinking) who regarded space and time as subjective or relational ideas, a matter of relative location and movement, and Newton who regarded them as absolute and independent aspects of objective reality, a backdrop in which physical events occur. Kant’s synthetic a priori assessment meant that both could be correct.

Enshrined in modern physics is the Theory of Relativity which tells us that when two observers (particles) are moving relative to one-another they will not agree on either the time that has passed between events, the space between objects, or even the chronological sequence of events. Furthermore, we also know that both observers are independently objectively correct. The observer and the observer’s position are an active ingredient of representation. This is a theory that puts past, present, and future into question, and demonstrates a physical link between time and space. Though common sense tells us that as phenomenal representations in our minds time and space are different things, as noumena in the noumenal world they are inextricably united, hence the physicists’ insistence on the greater scientific precision of the blended notion of spacetime. By the same token the fact that clocks can show predictably different times depending on their spacetime history seems to place spce and time clearly withing the physical world. But again, we perceive the world in four dimensions but perhaps this is just another limitation of our ‘spectacles’, can we ask if the noumenal world has more? The importance of the observer in scientific experimentation is found in the Heisenberg Uncertainty Principle also possibly quantum mechanics and quantum entanglement. These and other ‘weird’ conclusions of modern physics suggest that Kant’s philosophy was more than idle speculation. There is still much to learn about space and time.

A priori modes of thinking: categories of the understanding (cognition)

To become knowledge our perceptions must be conceptually grounded. In Kant’s words the ‘Transcendental analytic is the dissection of the whole of our a priori (both analytic and synthetic) knowledge into the elements of the pure cognition of the understanding.’ The categories are logical functions that make thought possible.

The Transcendental Analytic

A person walking towards us constantly changes appearance (in lighting, movement, size etc.) but we retain a distinct idea of that person all the time. This stable concept is a product of our cognition, not our perception. The Categories[11] of cognition make up our reasoning faculty, including the foundational principles of logic that allow us to order and classify. Phenomena are perceived through the filter of the Categories. Kant’s transcendental logic comprises the laws of understanding that select, coordinate, and unify the Categories of perception into the concepts of thought.

Kant described twelve filtering Categories in four groups of three. The four groups were the categories of quantity, quality, relation, and modality. These are filtering categories that process the contents of our mind, including the sensory information that flows into our minds after it has been internally processed into space and time (these are an elaboration of 10 Categories developed by Aristotle – Categories that, for the most part, make up the assumptions of logic).[9] These Categories, according to Kant, are the mental lenses that make thinking about the world possible: they are the minimum number of Categories needed to make objective (devoid of all empirical content and operating with necessity and universality) judgement possible: they are universal to human cognition. He called them ‘pure concepts of the understanding’ . . . ‘pure’ in the sense that they are not derived from empirical evidence.


Quantity: unity, plurality, totality
Quality: reality, negation, limitation
Relation: inherence/subsistence, causality/dependence, reciprocity
Mode: possible/impossible, existence/non-existence, necessity/contingency


For Kant our understanding is the active structuring of mental representations. Imagination, Kant said, is able to hold concepts, compare representations, and perform all those functions necessary for synthesis. Judgement is knowledge about objects derived from concepts so the statement ‘this is a chair’ is a judgement based on the concept ‘chair’ and the sensory intuition of what ‘this’ refers to. The range of concepts we can generate is limited only by the power of our imagination although the application of a concept is restricted by the possible representations: we can imagine a unicorn even if it does not exist in the world. This is important. Kant is an idealist only in the sense that he thinks the mind interprets the world, it does not constructs it.

Principle 4 – the mind does not construct the world it interprets it.

Science, Idealism, & objective reality

If we can only experience the external world of noumena indirectly as structured mental representations (Kant’s phenomena or empirical intuitions) then how is objective knowledge possible? How can we overcome the subjectivity of perception and cognition to obtain an objective view of the world? This dilemma, well known in philosophy, is called the egocentric predicament.

Transcendental Idealism and Empirical Realism

Idealist Irish Bishop Berkeley had a saying esse est percipi (to be is to be perceived) and he argued that because we can only know the world through mental representations then that is all we can know: there can be no mind-independent world. This philosophical idealism is expressed in the question ‘if a tree falls in a forest and nobody is present to hear it, does it make a sound?’ There are degrees of doubt about the external world with solipsism denying the material world altogether (only the self can be known to exist): no mind, no sound of tree falling. Against such views were the materialists and naturalism, the view that there were no supernatural forces in the world and mind is just another, albeit special for us, manifestation of physical matter. This presented another philosophical dualism, the Idealist vs the Materialist (as expressed in the witticism ‘No matter, never mind’).

Kant was no idealist in this sense: he believed that the science of his day was amassing valuable and reliable objective knowledge about the external world. Science, through its power of prediction and its technological spin-offs, had amply demonstrated its reliable link to the material world, to noumena. Kant was an empirical realist who maintained that, though we cannot know the external world directly, this does not mean that we cannot know it at all: he believed that there was an external world that could be investigated scientifically using observation and experiment and that this could uncover what he regarded as necessary truths, like Newton’s law of gravitation. In his own words, he was ‘both a transcendental idealist and an empirical realist’.

So how did Kant argue that objective knowledge was possible?

Kant could not use empirical knowledge to demonstrate the possibility of empirical knowledge, so he used what he called a ‘transcendental deduction’:

1. Only if A then B
2. B, from which we can infer . . .
3. A.
(Where A = necessary conditions to have an experience B, B = experience B).

Rather than adopting a philosophical stance of skepticism and doubt, like Descartes and Hume, Kant claimed that it was beyond doubt that we obtain knowledge of the world, the important question then becomes ‘How is this possible?’

Kant asserted that it is the combined Categories of perception and cognition that make this possible. Judgements of perception (like ‘honey is sweet’) are particular and subjective but judgements of the understanding which are common to all humans, the concepts of cognition that allow us to synthesize and unify, allow us to make judgements that are both necessary and universal, that is, objective. Intersubjective agreement becomes possible even though each individual has their own ‘reality’ – the shared form of human cognition, the general human point of view, enables common (objective) understanding.

Kant distinguished between the transcendentally real (the noumenon or thing-in-itself, the ding an sich) and the transcendentally ideal (the phenomenon). He then divided the transcendentally ideal into the empirically real (like an oasis or a straight stick) and the empirically ideal (like a mirage or a stick that looks bent when placed in water). We are inclined to challenge Kant’s idea of the empirically real as being subjective, but Kant points out that though it is not possible to access the transcendentally real directly we can still make empirically real judgements. We assume that noumena are the objects of science but the objects of science e.g. atoms, species, trees, and test tubes are given to us phenomenally. Though the phenomenal world is the limit of our direct knowledge, science can assist us in an indirect understanding of the nature of phenomena, the things-in themselves. In spite of all this apparent subjectivity our cognition is grounded in empiricism – it is ‘subjectivity that works’.

This is a subtle point. For Kant our phenomenal knowledge is both transcendentally valid and objectively real. Space and time are empirically real, but they do not represent properties of things in themselves. Kant draws a further distinction between empirical objectivity and transcendental subjectivity. Space and time have empirical objectivity since they are a necessary precondition for experiencing (empirically) the world objectively: and, since they are inner Categories that are ‘given’ they have transcendental subjectivity.

Again, for Kant the external world may be a human interpretation of the world consequent on our biology, but it is not an illusion – that is abundantly clear when we bang our heads on something. He is fully aware of the move from the philosophical claim that our direct experience is only of mental appearances to the extended claim that there is no external world.

Kant believed we can only appreciate our inner sense when we also have knowledge of an outer sense. We understand that more clearly today in a more evolutionary light, as part of the evolved organism-environment continuum that allows us to persist.

Principle 5 – Reason, our understanding spectacles, allows us to move from the particular and contingent of our perception to the general and universal of our cognition. The world is given coherence by applying universal principles to particular events


For Kant’s critical discussion of causality see Explanation and causation

Metaphysics & the limits to reason

The Transcendental Dialectic

Kant believed that he had demonstrated that knowledge extends only to phenomena – to things ‘as they appear to us’ – and beyond that we cannot know or say.

In the transcendental dialectic he asks if a rational metaphysics is possible.

In Kant’s day, as at other times in history, philosophy was criticized for its descent into sterile debate. In contrast to science, philosophy didn’t get anywhere as philosophers built their various grand systems of the world. Kant wanted to resolve this issue, once and for all, by establishing the limits to both knowledge and metaphysics.

Kant concluded that the presumed absolute laws and truths of science (Hume’s empirical matters of fact) and logic (Hume’s relations of ideas) are, paradoxically, both limited and relative: they are limited to the field of actual experience and relative to our human mode of experience. He had concluded that in the act of experiencing the world, objects in the world provide stimuli to our sense-perception which are moulded by our human Categories of perception and cognition. Scientists assume they are dealing with objects or ‘things in themselves’ rather than perceptions, sensations, concepts, beliefs, and ideas. Since neither science nor religion can know noumena directly they must both therefore fall back on hypotheses. If a judgement is necessary and universal (a priori – Kant’s ‘pure concept of the understanding’ – a rational concept not based on experience) then it must be metaphysical. Since it is beyond perception such a concept must be transcendental (this time in the sense of being beyond the limits of understanding), an empty concept. Transcendental ideas like God, freedom, a first cause, noumena are all outside space and time and therefore unknowable. Metaphysics therefore cannot be about proving the existence of transcendental objects like God.

Kant’s position here might be interpreted as an attack on theism but Kant saw it differently. He was a religious man and a declared part of his philosophical project was to ‘do away with knowledge in order to make way for belief’.

He concluded that what cannot be resolved by experience or pure reason is of little intellectual interest. The whole point of speculative metaphysics had been to transcend experience thus gaining knowledge of the noumenal realm. Kant pointed out that only reason could do this and he had demonstrated that such speculations, about the world, god, and the self, were illegitimate, simply impossible.

Looking beyond our perception (senses) and our understanding (cognition) is to look beyond possible knowledge: it is trying to reach past phenomena to noumena.

The a priori categories (of the transcendental aesthetic and transcendental analytic) are categories of perception and cognition . . . they are not categories of reality.

That is, we cannot have metaphysical knowledge, only metaphysical belief.

The question then falls back on what counts as justifiable belief. Kant then tries to demonstrate how important metaphysical claims devolve into paralogisms (claims that are beyond logic) or antinomies (in which cases both for and against a proposition can be proven).


Kant considered that mathematics must be a priori knowledge because it is both necessary and universal. That is, 7 + 5 = 12 must be true (it is necessary), and it must be true without exception (it is universal). Less convincingly he argued that it must also be synthetic because because the concept ’12’ is not contained in the concept of ‘7 + 5’. A geometric example would be the analytic truth that a triangle is a three-angled figure and the synthetic assumption that the internal angles of a triangle add up to 180 degrees which is an informative addition to our knowledge. For Kant mathematical statements, though analytic, are also synthetic because they are ampliative, the predicate is not contained in the subject. Kant’s view that geometry is synthetic a priori runs against the analytic school that regards maths as logic.

Controversially, Kant maintained that without the concepts of space and time we could have no arithmetic or geometry because they presuppose the pure intuitions (spectacles) of space and time. Geometry is the science of space and arithmetic the science of time. All arithmetical operations are sequential and sequentiality presupposes time. Similarly geometry presupposes space. These views gave rise to the school of mathematical philosophy known as intuitionism.[7]

Laws of nature

Kant committed himself to the highly contentious view that the laws of physics derive from the human mind. ‘Thus the order and regularity in the appearances, which we entitle nature, we ourselves introduce. We could never find them in appearances, had we not ourselves, or the nature of our mind, originally set them there’ and ‘The understanding is the lawgiver of nature’.

Thus the laws of nature are not ‘out there’ in the things-in-themselves, but only as determined by the Categories of understanding. Laws of nature have objectivity by consensus, through intersubjectivity, as communication between many individuals. Ultimately, the Categories provide laws a priori under which so-called ‘natural laws’ must sit: the Categories are our limits to knowledge and therefore the limits to our experienced world. Reason finds its own grounding in the world since it only perceives that which it produces after its own design, that is, in the objects it sets out to explain. We only intuit a sense of orderly lawfulness, causality, space, and time.

Kant was precise in his use of language. Sensation was unorganized stimulus. Perception was spatio-temporally organized sensation. Conception was organized perception. Mind was organized experience. Science was organized knowledge. Wisdom was organized life. For Kant the order of the world exists through the ordering of the mind, not because of laws in the world itself but because the laws of thought must conform to the mental structuring that is applied to all experience. The thought that brings us the world is itself ordered, and the world can only be known to us through the filter of this thought. Even so there must be order in the world since we have developed the habit of association, so for example, if we always associate fire with heat then this attaches to an object in the world. Even so Kant insisted we consider the possibility that the synthetic structuring predisposition of the mind (the unity of our consciousness) presents us with a single deterministic universe (objects with mass, location, and velocity) in overall causal interaction.

This seems a highly unlikely claim. After all, Newton’s universal laws themselves have been superseded. But if it is true that our cognition of time, space, and causality are innate predispositions of the mind then they are rules by which we must necessarily and universally understand the world. Today we find such an apparently subjective to solipsistic view near-impossible. However, Kant does indicate that there may be correspondence between inner and outer worlds.

Though we cannot take our ‘spectacles’ off to reveal the world as it truly is we can, as it were, get closer to noumena than Kant was prepared to concede. One example might suffice.

Botanists have always tried to put order into the botanical objects of the world, the noumena that we call plants. Part of this process has, of course, been the constant revision of what comprises the cognitive category ‘plant’. Cognitive groupings of plants were at first related to their human use as medicines before greater emphasis was subsequently placed on the similarities and differences of their structures, that is, differences beytween plants themselves, minimising their relationship to humans. In trying to understand plants better as noumena groupings became more and more dependent on technology, employing the structures revealed by microscopes, chemical analysis, and other technology. Today all organisms can be placed into groups according to the structure of their DNA, the chemical that carries within it the volutionary history of the species. We might never know plants as noumena, but analysing DNA is an improvement on groupings based on medicinal or food values. When I classify a plant into a different group based on its genetic composition I have reason to believe that I am refining plant classifcation in a way that more closely reflects my taxonomic intentions than if I based it on characters apparent to the naked eye. The mind uses innate categories but can create additional categories that have greater precision. Categories can be harnessed to inspect themselves: that is the object of science.


Modern science, arising out of the Scientific Revolution, was spearheaded by the remarkable achievements of Englishman Isaac Newton (1642-1726/7) who created a coherent synthesis of our understanding of gravity, mass, force, and motion. Kant was an admirer of Newton and, as an afterthought during the CoVid19 pandemic of 2019-2020, I’d like to share with you Newton’s account of the circumstances of some of science’s most transformative breakthroughs ever – achieved when his university, Cambridge, was shut down during the plague, giving him time to pursue his own interests!

Isaac was born prematurely in 1642, his father, an illiterate farmer, dying three months before Isaac’s birth. When Isaac was three, his mother remarried to Reverend Barnabas Smith leaving young Isaac in the care of his maternal grandmother. Isaac disliked his stepfather, and he also resented his mother for marrying him. From the ages of 12 to 17 he attended The King’s School, Grantham which taught Latin, Greek, and probably mathematics. After this he moved to the family home at Woolsthorpe-by-Colsterworth, Lincoln, where his mother, now widowed again, tried to persuade him to become a farmer. However, he returned to school and, motivated partly by a desire for revenge against a schoolyard bully, he became the top-ranked student noted for his construction of model sundials and windmills.

Aged 19 he entered Trinity College, Cambridge, at first paying for his education by being a valet. The college taught the philosophy of Aristotle, though Isaac also read about Descartes, Galileo, and Kepler.

In the summer of 1665, over 350 years ago, he had just received his B.A. degree as an unremarkable student when the university was evacuated as a precaution against the Great Plague. This was the last major English outbreak of bubonic plague and it killed about one quarter of London‘s population only one year before the Great Fire of 1666 that burned most of the homes inside that Medieval city’s walls.

The epidemic released the young Newton from his university studies for two years, allowing him time to reflect on his current interests. The 23-year-old moved into his family home, the farmstead at Woolsthorpe, and it was during this period that he discovered the Differential Calculus, the Composition of Light, and the Laws of Gravitation. He later recalled his discovery of the laws of gravitation as follows:

‘In the same year (1666) I began to think of gravity extending to the orb of the moon . . . deduced that the forces which keep the planets in their orbs must be reciprocally as the squares of their distances from the centers about which they revolve; and thereby compared the force requisite to keep the moon in her orb with the force of gravity at the surface of the earth, and found them answer pretty well. All this was in the two plague years 1665 and 1666, for in those days I was in the prime of my age for invention, and minded mathematics and philosophy (science) more than at any time since.’

Newton’s law of universal gravitation states that a particle attracts every other particle in the universe using a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them. Newton called the process he used to derive this law ‘induction‘ and it became part of the classical mechanics of his Philosophiæ Naturalis Principia Mathematica presented in 1686 to the Royal Society and published on 5 July 1687.

In 1692 he wrote to a friend:

‘That one body may act upon another at a distance through a vacuum without the mediation of anything else, by and through which their action and force may be conveyed from one another, is to me so great an absurdity that, I believe, no man who has in philosophic matters a competent faculty of thinking could ever fall into it’

He never assigned the cause to gravitation, refusing to formulate a hypothesis because to do so would not be sound science. Much later Einstein’s General Theory of Relativity described gravitation as a property of curved spacetime rather than a force operating between bodies: energy and momentum distorted spacetime and it was this that accounted for the motions of light and mass that were consistent with existing observations.

On returning to Cambridge he was elected a fellow of Trinity and, as such, was required to become an ordained priest, though this was not enforced in the restoration years when an assertion of conformity to the Church of England was sufficient. He was elected a Fellow of the Royal Society (FRS) in 1672.

What were you doing . . . or what will you be doing, at the age of 23?


This has been a difficult article that needs a summary.

Hume’s skeptical empiricism and views on causality awoke Kant from his ‘dogmatic slumbers’, impelling Kant to spend 11 years developing a refutation, the Critique of Pure Reason. His response was a brilliant rebuttal to skepticism and extreme empiricism.

His philosophy

Kant’s reputation as perhaps the greatest philosopher of the modern era rests on his coherent synthesis of the European philosophy of his day. He provided an epistemology that included a closely-argued account of the bounds of reason, understanding, perception, cognition, experience, imagination, knowledge, science, metaphysics, morality, religion, and reality.

He answered the question – ‘How is synthetic a priori knowledge possible? by claiming that all knowledge of the world is only of appearances in the sense that all we can know of the world is of phenomena (things that have conformed to our modes of experience) and not of noumena (independently real things in themselves  that are independent of our modes of experience). He challenged the prevailing 18th century view of humanity as a ‘spectator’ to reality.

Limits to knowledge

Kant agreed with the Empiricists that knowledge arises in sense experience but he added that this form of knowledge, to be meaningful, must be synthesized into a conceptual framework by our two innate cognitive capacities, the Categories of perception (percepts – organised in space and time) and cognition (concepts – via 12 logical Categories based on quality, quantity, relation, and mode). To obtain knowledge the claims of both empiricists and rationalists must be taken into account.

For all the dense prose and technical words Kant’s actual claims were direct and clear. Reason was the integration of experience into a coherent conceptual framework through the structuring processes, the Categories, of the mind. The Categories are necessary a priori rules that prevent our experience and knowledge from being a haphazard and arbitrary ‘buzzing confusion’.

Part of Kant’s declared project was ‘to establish knowledge in order to make way for belief’. He tried to demonstrate the bounds of human knowledge – to discover what was possible given the mental tools that we have at our disposal. All knowledge of the world, he concluded, was knowledge of phenomena. Realms of knowledge outside these limits, like religion and metaphysics, were inaccessible. This did not mean that statements about religion were meaningless or mistaken: religion can be accessed by faith – but it cannot be a form of formal knowledge as he defined it.

For Kant metaphysical knowledge must be synthetic a priori knowledge and this, therefore, was the task of the philosopher – to discover synthetic a priori truths.

Subjectivity – the egocentric predicament

Kant’s philosophical Copernican Revolution was the challenge to the Enlightenment view that humans are detached spectators surveying an objective world. Instead he argued that, of biological necessity, we cannot step outside ourseleves: our outlook on the world must be a human outlook. We are therefore participants rather than spectators. Kant brought a new urgency to the question ‘Does nature conform to our thought or does our thought conform to nature?‘. On the one hand he argued against a totally idealist or solipsist claim that reality only exists in individual or collective minds, but also against the simple claim that the mind is a passive mirror or blank slate (Locke) reflecting or recording reality by giving us a God-like or non-human detached view of reality (noumena). We cannot achieve a view of the world from no-time, no-where, and no-body. Kant’s point is acknowledged philosophically as the ‘egocentric predicament’.


Science in the 18th century was regarded as ‘A systematic body of principles that grounded truths that were both universal and necessary’ (see Dan Robinson’s video). In simpler terms – science was absolute truth and humans were passive observers of the factual world.

Kant viewed science as a robust form of knowledge. He did not approach his intellectual task from a position of extreme skepticism like Descartes and Hume. Rather, he refreshingly accepted the self-evident power of scientific knowledge, and tried to move forward by asking ‘What are the conditions necessary for this to be the case?
Some of his conclusions about science have failed the test of time (such as the absolute nature of Euclidian geometry) but his mode of thinking has continued to inspire.

Today’s idea of science as objective knowledge with a clearly defined and domain-specific method has been effectively abandoned for a view that regards it as socio-economically embedded and theory-laden, more a web of interconnected ideas that sometimes move by changes in perspective (paradigm shifts) rather than sudden falsifying discoveries and, yes, entailing a degree of subjectivity as suggested by Kant’s ‘spectacles’.

Kant forced us to consider the possibility that space, time, and causality, rather than being properties of the world, are conditions of our experience.

Thought & language

Cognitive scientist and linguist Steven Pinker notes in ‘The Stuff of Thought’ (2008) that we can mistakenly assume that language structures our thought and our world – that we think linguistically. However, in his view, thought is prior to language as an internal dialogue that he calls ‘mentalese’.

Pinker also describes how key scientific concepts of space, time, matter, and causality are embedded in everyday language. Nouns express matter as stuff or substance and things extended along one or more dimensions. Verbs express causality as agents acting on something. Verb tenses express time as activities and events along a single dimension. Prepositions express space as places and objects in spatial relationships (on, under, to, from etc.).

These are, of course, key intuitive categories isolated by Kant. This language of intuitive physics may not agree with the findings of modern physics but, like all metaphor, it helps us to reason, quantify experience, and create a causal framework for events in a way that allows us to assign responsibility. Pinker states that ‘Language is a toolbox that conveniently and immediately transfers life’s most obscure, abstract, and profound mysteries into a world that is factual, knowable, and willable‘.

Depth psychology

Kant’s description of structuring mental processes, unconscious and therefore hidden from direct inspection, would be a stimulus to the development of psychology notably the Principles of Psychology (1890) published by American William James, although we associate the unconscious mind more with people like Sigmund Freud (1856-1939) and Carl Jung (1875-1961).

Logical positivists

Kant’s definition of what constitutes knowledge was taken up and reconfigured in the 20th century by the logical positivists who argued that, in order to be meaningful, propositions must be either analytic or empirically verifiable. This ‘verification theory of meaning’ was abandoned when it became clear that much meaningful discourse was not of the kind the positivists claimed. However, this prompted intensive discussion in the philosophy of science and a critical examination of the role played by both verification and falsifiability (notably in the work of Karl Popper).

Cognitive science

To a degree, the questions posed by Kant have been taken up through cognitive science and the studies of perception and cognition. Modern science emerged out of philosophy with Newton in the 17th century accentuating its growing independence as philosophy began an attack on its own grand systems. This self-doubt has continued in philosophy, raising questions about its value and role, many philosophers showing deference to science as a source of knowledge and accepting the role of philosophy as a ‘clarification and clean-up service’.

The innate

Kant would likely have resisted any of the connections that I make here between reason, intuition, and the biology of the mind. In Kant’s day, and for many philosophers today, such a jump is simply unjustified – there can be no simple connection between these philosophical categories and physiological states.

However, with the mind, as with the body, questions of innateness continue to play a major role in intellectual life in the constant battles fought over nature and nurture. We see this, for example, in the innate universal grammar proposed by linguist Noam Chomsky. Numbers and percentages are now being allocated as measures of how much is ‘internal’ and how much ‘external’ in regard to particular traits. Rationalism has been reinterpreted in many ways, notably in the modern guise of ‘psychological nativism’ or ‘innatism’, the idea that certain capacities are hard-wired into the brain at birth.

Kant & Darwin

Kant lived about 100 years before Darwin published his theory of natural selection. For all its brilliance Kant’s project was completed at a time when the place of God was still central to any account of the scheme of things. Nearly a century before Darwin’s theory of natural selection people believed in the immutability of species, that each species was created uniquely by God, and that the earth was just a few thousand years old. Though there were similarities and differences between organisms there was no necessary underlying biological commonality, no sense of all life developing from a common ancestry.

Humans were regarded by many as being apart from the rest of the biological world: they were unique and special parts of God’s Creation. Humans, and especially European humans, had a very strong sense of their moral superiority and difference from other humans and lowly animals. In Kant’s day there was still only the grudging acceptance of the Copernican idea of a heliocentric solar system that relegated humans to the periphery of the physical universe.

Today we have the advantage, that Kant did not have, of the theory of natural selection. From an evolutionary perspective the more accurately our reason maps or adjusts to the world (Kant’s noumena) the more likely we are to survive, bearing in mind that our map of the world will be a distinctly human map. For this reason alone it is reasonable to expect our phenomena to match the world’s noumena and, since Kant’s time, scientific research has suggested the validity of this assertion while at the same time our scientific investigation of time and space have vindicated Kant’s warning that time and space are a human gloss of the world. Science can test noumena through our spectacles and make progress even though it can never know them directly, and the notion of a ‘true’ map of reality just does not make sense by Kant’s ideas. Science helps us gain a better understanding of perception and cognition, helping us to reduce biases of interpretation. Some might ask if it is possible for the Categories to be culturally encoded, or to consider ways in which they might have evolved.

Today we can confidently assert, as Kant could not, that the kind of world we experience in our minds is a consequence of our evolutionary history. Our minds and the processes that go on in our brains are a consequence of the historical evolutionary interaction between organism and environment that is evolutionary history. From an evolutionary perspective the more accurately our reason maps or adjusts to the world (noumena) the more likely we are to survive, bearing in mind that our map of the world will be a distinctly human map. For this reason alone it is reasonable to expect our phenomena to closely match the world’s noumena. Since Kant’s time scientific research has suggested the validity of this assertion. There is a sense in which Kant anticipated natural selection with its crucial emphasis on the interaction between organism and environment. Rationalists saw knowledge as emanating from within the organism, Empiricists that it could only arise from without. Kant’s vision of our mental world, like natural selection itself, blended the inner and outer: it integrated mind and the world.


Today we can ignore the elevated position of humanity in the scheme of things and, recognizing ourselves as animals among other animals, also recognize that our experiential reality is no more ‘real’, has no more validity, than that of other animals. Our reality, like that of other sentient animals, is a product of our unique evolutionary history of adaptation to past environments, the inherited structuring of our minds.

But does this lead to a kind of reality relativity? Does this mean that reality itself is relative … humans have one reality, bats have another: I have my reality, you have yours. Though a bat and human might perceive and interpret a fly in totally different ways they are both dealing with the same object, an object that is not just a mental representation. Though the ‘reality’ of one sentient organism has no precedence over the ‘reality’ of another, there are nevertheless important cognitive differences between humans and other sentient animals.

One difference lies in the processing power of the human brain which is greater than that of all other sentient creatures. If we re-express Kant’s thesis in post-Darwinian terminology we might say that we arrive in the world with minds that are pre-programmed through the evolutionary process to structure the world in a way that makes sense, and which allows us to survive and reproduce. Ultimately it would seem that this had major evolutionary consequences. Perhaps in early human evolution these advantages were not great. The major difference that allowed humans to dominate the world, it appears, lay in the human capacity to accumulate and store culturally acquired knowledge that could be transmitted through language – with all the consequences that this entails.

For Kant, our knowledge of the world can only be of phenomena (our interpretation of things) not noumena (the things in themselves). Metaphysics cannot explore the world of noumena.

Knowledge – the epistemological project

Aristotle regarded metaphysics as the study of being (ontology) whose mode of enquiry was epistemology. He had declared that ‘nothing is in the intellect which was not first in the senses’. Liebniz, about 2000 years later, added the riposte ‘except the organizing mind itself (the intellect)’. Taken together these two statements summarize Kant’s philosophy (Liebniz, who died 8 years before Kant was born, indicates here a possible influence on Kant’s notion of the synthetic a priori).

Kant’s summary epistemology and reply to Hume was that ‘Knowledge begins with the senses, proceeds then to the understanding, and ends with reason. There is nothing higher than reason.’ Kant’s characterization suggests perception as the passive reception of sensory information, understanding as the non-judgmental application of the Categories to give meaningful structure to experience, and reason as the application of judgment to understanding.

‘Understanding’, for Kant, is the faculty concerned with actively producing knowledge by means of concepts. It is the synthesizing capacity of the brain that gives us a logical perspective, which enables us to order both concepts and percepts. It allows us to combine concepts with intuitions in order to produce empirical knowledge. Without it, science could not proceed.

. . . And that is why scientists need to think about what Kant, both philosopher and scientist, had to say.

Direct (naive) realism (common sense realism)

If as Kant claims, we can never perceive objects (noumena) as they are in themselves, but only as they appear to us (phenomena) then this suggests a veil or barrier between ourselves (perceiver) and the ‘real’ world (object), sometimes called the ‘representative theory of perception’. It is as though we can only know about someone by seeing their portrait. The representative theory of perception is generally attributed to Locke.

Among those criticizing idealism and representationalism were the influential common sense philosophers of the Scottish Enlightenment: Thomas Reid, James Beattie, and John Oswald.

Philosopher-scientist Thomas Reid had followed Adam Smith to the chair of moral philosophy at Glasgow University. Reid claimed that the primary objects of sense perception are physical objects, not ideas in the mind. We have direct, not indirect, access to the external world. What occurs in the mind is a presentation not a representation. All humans experience the external world directly, immediately, and in an unquestioned way – without mediation, filters, or the operation of reason. It is not representations that have secondary qualities of colour, smell, and taste, it is the objects themselves. These Scotsman believed that idealism of any kind was a philosophical indulgence unavailable to humanity at large. That direct experience is a precondition of existence ‘necessary to all men for their being and preservation’ and it is only because we have this unquestioned world of common experience (common sense) that communication becomes possible.

This theme is pusued in the article Reality and representation.

After Kant

Kant distilled philosophical history into a clean-lined intellectual monument. Subsequent work would inevitably break it down once again, reconfiguring the bricks into a different building. Today we are in the middle of the process of demolition, not even sure about the possibility or value of the monument itself. There appeared to be a logical inconsistency in Kant’s claim when he agreed with Hume that causality was a product of the mind, while suggesting that noumena ’cause’ phenomena. Kant appeared at times to move close to a truly idealist position by claiming that because we cannot experience objects directly then we cannot test our experience of them at all, that noumena lie completely beyond the realm of knowledge while he, at the same time, claimed the possibility of objective knowledge.

Absolute idealism. Since noumena were not doing any explanatory work (all knowledge applied to phenomena), then noumena were abandoned by Fichte (1762-1814), Hegel (1770-1831), and the German Idealist school (Absolute Idealism).

The linguistic turn. Over time Kant’s conceptual schema was increasingly regarded as culturally bound that non Europeans used different conceptual schemes embedded in different languages. There are many languages, as sets of symbols used to describe nature, but there are no objects describable in the abstract, independent of the language of description. So, the Categories were increasingly treated in historical and cultural, not universal, terms and language became the focus of attention, American philosopher Quine quipping that ‘Ontology recapitulates philology’ (a play on biologist Haeckel’s ‘ontogeny recapitulates phylogeny’). Conceptual schemas became mental software rather than mental hardware.

Pragmatism. Kant’s claim that we can only know phenomena (the world as it appears to us, not the world as it is independent of us) challenges the idea of an absolute reality, the view that our (scientific) ideas correspond to or ‘mirror’ the world. He opened the possibility of a world where his transcendental ideas were not ‘true’ but ‘useful’. They could, for example, prompt further enquiry. This opened the door to American pragmatism.

Today’s scientists would probably place more intellectual trust than Kant in the possibilities presented by our ‘indirect’ knowledge. Perhaps, in retrospect, Kant’s emphasis lay heavily on the way the mind constructs the world in spite of his insistence that it was the way it interprets it?

Fellow countryman Schopenhauer (1788-1860) was a major critic of Kant. If the world is differentiated according to human Categories then is it possible to think of how it might be devoid of this interpretation (The world as Will (energy) and Idea).

Subsequent philosophers, known as the Neo-Kantians, questioned the a priori nature of Kant’s Categories, while the security of the analytic-synthetic distinction has been challenged in the 20th century by American logician Quine. The number and character of the Categories themselves has also been challenged.

Key Points

The Critique … was Kant’s answer to Hume’s skepticism and empiricism in which he demonstrates the possibility of prior knowledge, of knowledge independent of experience. In so doing he claims that Hume’s ‘Fork’ creates a false dichotomy because knowledge requires both experience and reason.

The Critique . . . was a study of the mental preconditions needed for experience to be possible.


  • He explored the limits and conditions for knowledge, and advanced our understanding of, among other things, space, time, substance, and cause – together with a closely-argued account of the bounds of reason, understanding, perception, cognition, experience, imagination, science, metaphysics, morality, religion, and reality
  • Kant claimed that our minds operate like colour lenses in spectacles. The colour we see seems to be a property of the world but it is added by the lenses. The mind filters our experience of the world in a similar way. Not only are we unable to see the world ‘as it actually is’ but the the world ‘as it actually is’ can only ever be what we as humans make of it, that is, the world ‘as we interpret it’. Today we would call Kant’s mental filter the innate cognitive structuring done by our mind to give us our human ‘reality’ (different from the reality of a fish or bat)
    • Kant divided the world into the noumenal world (what exists in the mind-independent world), and the phenomenal world (which is what we experience through our innate cognitive filter)
      • Our human interpretation of reality is not an illusion: it is not in some way mistaken or incorrect but it is nevertheless an interpretation, albeit an interpretation provided by an extremely powerful cumputing device (our brains). But it is an error to think of the noumenal world as a separate unknowable world. All we have are phenomenal interpretations. The problem is that we assume that behind our interpretation is a ‘reality’ or ‘thing in itself’ as in ‘actually is’. Such an entity is not ‘unknowable’ or something of which we can only have ‘indirect knowledge’, it simply does not exist
        • Among the necessary preconditions for experience (the innate faculties of the mind) Kant included those of perception (space and time) and those of cognition (twelve faculties under the general headings of quantity, quality, relation, and mode). This is knowledge that must be presupposed if we are to have any coherent experience of the world at all. Knowledge emanating from this source is expressed in synthetic a priori statements, that is, language describing the world that does not derive from experience of the world. Kant included here: causation, mathematics, logic, metaphysics, and religion
          • Note that Kant did not deny the existence of space and time, he simply pointed out that we can only experience them as phenomena, not noumena – that is, as configured by our innate cognitive filter which is the instrument or mode of our understanding
            • Note also that scientists explore the empirical properties of space and time while Kant is examining what is necessary for us to have any experience at all – whether it be of space, time, or indeed, science itself. Observation and cognition presupposes some means of structuring and categorizing observations and experience. Newton and Einstein did not modify, disprove or confirm what Kant had said: they were only incidental to what he was attempting. Modern technology has allowed us to investigate noumena in ways that Kant could not have imagined – this will be discussed in the next article Appearance and reality.
              • We can learn from Kant and place a modern gloss on his work with Principle 5 (below) which we can carry over to future articles. What is the relationship between Kant’s cogitations and modern science?
                • Our mental representations of the world are not illusory or mistaken (except for those that are empirically illusory), but they are uniquely human representations constrained by the biological limitations of our human sense perception (the perceptual capacity or range of our sight, hearing, taste, smell, touch, and other possible sensory input) and the inherited limitations of our human cognition. The capacities of our perception and cognition no doubt reflect those factors that were important for the historical survival and reproduction of our species: they are an adaptive psychological mechanism. What we can know of the world is partial – our best interpretation of the phenomena, including the extension of our perception and cognition using science and technology. There can be no world ‘as it actually is’ or ‘thing in itself’ beyond our interpretation. Our human-eye experience of the world has been extended by science, the application of the computing capacity of our human brains to the knowledge of the world obtained by the technological extension of our biologically-given senses. This this will be discussed further in the next article
                  • Organisms blend with their environments in an organism-environment continuum.

                    Media Gallery

                    Professor Arthur Holmes introduction to Kant’s philosophy points out that skepticism arose out of the concept of cause and effect and that, for Kant, all metaphysical concepts, like that of causation, arise as synthetic a priori propositions or, as we might want to say, innate intuitions as the first principles for deductive knowledge. The question then arises – ‘How are such propositions possible?’ Kant mounted a compelling challenge to both his rationalist and empiricist philosophical predecessors.

                    Locke, Berkeley, & Empiricism: Crash Course Philosophy

                    CrashCourse – 2016 – 9:51

                    The Broader Philosophical Context of Kant’s Critique of Pure Reason – Dan Robinson

                    Philosophy overdose – 2019 – 44:10

                    Beginner’s Guide to Kant’s Metaphysics & Epistemology

                    Philosophy Tube – 2016 – 5:55

                    Concepts, Judgment, & Kant’s Transcendental Deduction of the Categories

                    Philosophy Overdose – 2019 – 40:16

                    A History of Philosophy | 51 Introducing Immanuel Kant

                    WheatonCollege – 2015 – 1:04:21


                    First published on the internet – 1 March 2019
                    . . . substantial revision – June 2020
                    . . . revised – October 2020
                    . . . minor edits – July 2021



                    Kant’s house in Königsberg – 1842
                    Artist – Friedrich Heinrich Bils
                    Courtesy Wikimedia Commons – Daube aus Böblingen– Accessed 7 January 2021

                    Kant's house in Königsberg – 1842
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