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CONTEXT

See the general introduction to plants and people which discusses the context of these two articles on classification in general and plant classification in particular as they relate to themes discussed on this web site. This is an article of generality that examines the way that we frame all human existence – the necessary context for all scientific and plant classifications. This article considers the way our minds operate on all the categories of experience before examining the way that we manipulate the categories of science in general and plant science in particular. It is suggested that our sophisticated methods of classification can be applied to not only plant-plant relations as in plant classification, but also applied more rigorously to the relationship plants-humans, and plants-environment.

Classification

– For a combined summary of this article and that on plant classification see the Epilogue –

Aristotle gave us a tentative foothold in the mystery of existence by telling us that the world consists of objects, their properties, and relations.

As we shall see, whether or not we regard his summation as arbitrary or unduly philosophical, it captured, in a succinct way, our intuitive way of experiencing and interpreting – of classifying – everything in the world.

The elements

The objects of the world – whether the objects of our mental experience, or the physical objects we see all around us, seem unproblematic (see the manifest image). However, to know what an object is, we must know how it differs from other objects in its properties and relations, and this is more obscure. Properties are located in the object. Medieval science treated properties as the ‘powers’ of objects and much scientific time has been devoted to their elucidation. Relations are the ways in which objects, the relata, stand to one other and they resemble properties by describing their objects, but properties are a special case of relations that involve only one relatum.

To discuss the diversity of classifications we might encounter we need a clear understanding of, and terminology for, their key elements.

Since classifications explore relationships the objects to be classified are called the relata. The features (properties) used to group relata are here called selection criteria. The representation of the relationship then becomes the classification system.

This is a highly flexible and abstract schema whose contents (objects, properties, and relations) all depend on the purpose of the classification. Objects in one classification system may be properties in another, and so on.

We are inclined to think of classification in terms of the scientific classification of physical objects like plants and animals. But we are constantly classifying and reclassifying everything in our experience as part of the process of orientating ourselves in the world of objects including mental categories, concepts, ideas, cognitive units, and physical objects.

This article examines classification in its most general sense and how this has been applied to plant classification (also considered in its most general sense). It then investigates what it means to establish a scientific classification, and if it is possible to establish a scientific classification of the relationship between plants and people.

The process

Our minds are constantly – both consciously and unconsciously – ordering the constant input of sensory and other information into what we understand as meaningful experience. This involves the segregation, filtering, comparison, and ranking of the objects of our awareness into the integrated and coherent whole that is our awareness of the world.

The units of our mental life (referred to here as categories) are combined and divided in various ways to become further categories in a complex web of related groups and sub-groups that are all available for further mental processing. It is with this complex mental ordering – classification in its broadest sense – that we orientate ourselves to the world.

Classification, in this general sense, is an agential activity expressed by all life. It is a process of discrimination that guides flexible behaviour as organisms adjust to both their internal and external environments while pursuing the ultimate goals of survival, reproduction, and flourishing. But only in humans has this evolved into a complex minded process, a consciously reasoned social activity that contributes to the progress of knowledge via the medium of language.  Scientific classification systems are just one example of the collective learning that informs our human orientation to the world.[13]

Taxonomy is, in effect, meta-classification: it is the study of the principles and procedures of classification. Viewed in this general way, classification and taxonomy together become the study of the relationships between the objects of our experience.

A distinction must be made between classification as the process of ordering, and a classification as a representation or product of this process (a classification system).

This article examines the foundations of all classification systems and how they relate to scientific classifications. It looks at their principles of construction, their operations, and their different structural forms. A follow-up article then discusses the special case of scientific plant classification which also investigates the possibility of a scientific classification for the relationship that exists between plants and humans.

Understanding the foundations of classification helps us to assess, in a more informed way, the service provided by our current scientific classifications.

Agency & purpose

In responding to the world in an agential, goal-directed, or purposive way, all organisms demonstrate degrees of flexibility in responding to, mirroring, or ‘representing’ the world. Most organisms do this in a mindless way but, as humans, we are accustomed to the mental representations that are a product of our conscious intentionality. This classificatory structuring of experience as a means of orientation to the world is different for each organism and determines its ‘reality’ (umwelt) – its species-specific sense of what the world is like.

There are an infinite number of ways for us to classify the vast number of mental objects that can potentially populate our minds; but we do not create random classification systems, using random categories arranged in random ways. As intentional organisms we create purposeful classifications that we use as tools to attain our goals. Because classifications are produced for a reason, they are a part of our reasoning faculty.

This provides a useful grounding principle for all classification systems. They reflect goal-directed interests and act as ‘decision-making tools’ as organisms adjust to their inner and outer worlds. In humans it is treated as the organization or grouping of categories into a formal classification system. In science it is most familiar as the biological organization of organisms into a system of hierarchically ranked and formally named categories.

Principle – classification in a general sense is one manifestation of the goal-directed (purposive)  orientation of living systems to their inner and outer environments. In humans, classification, as an intentional mental process acting as both an unconscious and conscious decision-making tool. Formal scientific classifications are public and progressive representations of the world – a part of collective learning.  

Mental processing

Eighteenth century German philosopher Immanuel Kant argued convincingly (against the views of many of his contemporaries) that the mind was not a passive ‘blank slate’ on which a person’s experiences are written. Rather, the mind contributes to ‘knowledge’ by actively structuring our experience. Kant even outlined in detail what it was that he thought the mind contributed to our understanding of the world. This amounted to our placing of all experience within the context of space and time (his transcendental aesthetic) and within logical structuring principles or ‘categories’ (his transcendental analytic – which uses the word ‘category’ in a different sense from that in this article).

The detail of Kant’s argument remains contentious but his claim that the mind frames our experience and knowledge is now widely accepted. To use Kant’s form of expression: the mind must have certain predispositions if we are to have any meaningful experience at all.

Since Kant, the whole field of psychology has opened up. It is now clear that our minds are constrained by our species-specific human biology in many ways. So, for example, we do not experience everything it is possible to experience, all at once. The flood of incoming sensory information is subjected to our mental processing which, by abstraction, reduces and organizes our experience. This is an essential precondition for us to operate effectively in the world.

The consequence of this mental processing is that we exist in a world that is unified, coherent, and meaningful . . . not a confusing maelstrom of competing sensations.

 

How do we humans convert the multitude of ideas in our minds, the constant influx of sensory information, our representations of physical objects etc. etc. into a coherent experience that allows us to live effective lives?

We can identify, in principle, at least four mental predispositions that are necessary for meaningful experience. All of these are context-sensitive: that is, the form they take is determined by the purpose or goal.

The mind categorizes the world into meaningful units of representation (mental categories); it focuses on a limited range of these objects at any given time resulting in foreground and background awareness; it groups these categories in various ways; and it ranks its categories and category groups in relation to one-another according to conscious and unconscious priorities.

These four processes of categorization, focus, grouping, and ranking occur in all cultures and are, indeed, necessary for our survival. They are part of the mental ordering of our experience that we associate with reason – an evolved and inherited biological characteristic that is part of our human nature.

Clearly this mental ordering of our experience is an evolved and inherited biological characteristic, a part of our human nature and the way human brains structure experience determines our species-specific ‘reality’, our experience of what the world is like. As an ordering process it is also a key ingredient of our uniquely human capacity for reasoning. Each of these processes is now considered in more detail.

These processes seem to occur simultaneously and with varying degrees of consciousness. At one extreme might be the way our eyes move intuitively towards the source of a noise, while their smooth integration when I am driving is a much more miraculous and complex fusion of the conscious and intuitive. The solving of a mathematical problem is a fully conscious process.

Each of these processes is now considered in more detail.

The mental process

Classification is a product of the agential activity of living organisms as they orientate themselves in relation to their environments. It is, however, most familiar to us as both a conscious and unconscious activity of intentional human minds.

Our common sense tells us that the world has a structure or mode of existence that is independent of ourselves. But, while we can hardly doubt the external world, its ‘true nature’ remains a matter of interpretation. We humans see the world as we do because this is a perspective (manifest image) that has allowed us to survive in evolutionary time. Thus, there is a biological purpose to the unique way we humans see the world (our perspective on the world’s objects, properties and relations) and that purpose is expressed in the biological axiom (survival, reproduction, and flourishing) which is expressed in our uniquely human sensory apparatus.

This gives us a grounding principle for all taxonomy: classifications are a product of agential activity with every classification serving a purpose or reason.

Principle – classification systems are decision-making tools. For humans they provide a representation of the world that is orientated towards a particular purpose or end

The mental process of classification itself can be a private, unconscious, and individual matter – an automatic mental computation as a process of discrimination – as when we orientate ourselves to the objects around us when driving. This can be compared to the mindless aspect of biological agency.

But classification can also be part of a public collaborative and progressive system of representation that contributes to collective learning – as when we construct the various classification systems used by science.

The article on classification examined the notion of classification in its broadest sense. It described the way our minds think in mentalese[5] and manipulate ideas using a mental process with at least four key computational ingredients: segregation, focus, grouping, and prioritization.

(a) segregation – divides the world of experience into meaningful operational units of thought called categories (concepts, or units of mental experience)

(b) focus – restricts our attention to a manageable number of categories (creating a foreground and background)

(c) grouping – arranges categories into groups using selection criteria that are meaningful in relation to desired objectives – the structure of groupings in particular cases is called the classification system

(d) ranking (prioritization) – arranges groups into a ranked order – which facilitates action or decision-making

Categories & Categorization

Categorization structures our world into meaningful representational units of experience (mental categories[11]), both those of cognition (concepts), and those of perception (percepts). These are the operational units of our mental classifications,[12] with the operational units of formal public classification systems a small subset of this totality.

The fact of individuation indicates some degree of discontinuity although this discontinuity can be a matter of degree. The way our minds convert the field of colours, textures, and tones that falls on our retinas into meaningful and discrete objects like cars, people, trees etc. is a breathtaking mental feat that we all take for granted.

It is these meaningful mental units of representation (categories) that we group in various ways to provide representations of the world that constitute our human experience.

If we are seeking clarity in our communication, then we need to feel secure about the categories that we are using. The most obvious way in which we communicate categories is by the use of words as names of various kinds. Unfortunately, name-categories denote meanings of greater or lesser generality and abstraction.

Consider the practicality/reliability/precision of the categories ‘my cup’ and ‘the chair by the window’. Then consider the categories ‘cup’ and ‘chair’ and then further general categories like ‘furniture’ and ‘crockery’. Names, as representations, relate to the world in complex and confusing ways.

The words we use, and their meanings, are categories in the web of knowledge. But the connections between the nodes in this web are of different strength. The meaning boundaries that we draw between related category words that we use to fragment and describe the world can vary in the semantic strength with which they bind to one-another. Meanings can merge to a greater or lesser degree with the meanings of other words, like streams of different strengths flowing into one-another. Consider the meanings of the words ’cause’ and ‘reason’ and how their meanings overlap. Other word categories are semantically clear-cut, technically precise, and secure (e.g. anther).

It is useful to distinguish two extreme kinds of category – classical and fuzzy.

Classical categories

Philosophers have sought clarity in communication by using categories that are clear and distinct. Plato wanted to establish categories for the external world that ‘carved nature at the joints’, by which he meant dividing the physical world into units that we believe exist in nature not just as representations in our minds.

Aristotle described what we now call ‘classical’ categories which had defining features (selection criteria) that were both necessary and sufficient. These defining features became known as ‘essences’, and the use of these classical categories was called ‘essentialism’. The value of these categories was that they were clearly defined, mutually exclusive, and collectively exhaustive – which did indeed provide a basis for clear and distinct ideas. This general point in relation to science and classification is conveyed by Oxford physicist David Deutsch who describes science as being concerned with ‘good explanations’ -these being explanations that are difficult to vary while still supporting their claims.

One obvious scientific example of classical categories is the Periodic Table of elements, each element defined by its atomic number. Though gold has many properties that can be used as categories amenable for a definition of gold, scientists are agreed that a definition of gold as the element having atomic number 79 is both necessary and sufficient: it is also precise and succinct. Before the advent of atomic theory other properties were used to define gold: atomic theory refined these older (fuzzy) definitions. Unfortunately, this degree of clarity occurs more frequently in logic and mathematics than in nature. But Aristotle reminds us of what constitutes a powerful intellectual category – the form of category that was part of his logical deduction that was so maligned during the Scientific Revolution.

The later Darwinian theory of natural selection supported the idea that the search for essences in nature was a vain hope. The scientific diagnosis of a species aspires to the ideal of the classical category, but evolution by descent with modification (rather than special creation) has made the possibility of providing ‘essential’ diagnoses of a species (as classical categories) a tall order.

Fuzzy categories

Most categories applied to natural objects (and the meanings of most words), are blurry; their meanings grade into the meanings of other categories at their boundaries.

As the philosopher Ludwig Wittgenstein pointed out, the notion of a ‘game’ is not a classical category. The many different activities that we call ‘games’ show a ‘family resemblance’ rather than a set of necessary and sufficient (essential) conditions. In other words, what we mean by ‘game’ grades into the meanings of other categories making it a fuzzy, rather than clear and distinct, category.

Awareness of the wide application of fuzzy categories in our communication has given rise to new conceptual clustering tools like those developed in fuzzy mathematics (fuzzy logic and fuzzy set theory which developed in the 1960s) in which objects may belong to one or more groups but to varying degrees. Psychologists studying ‘category learning’ investigate the possible ways our brains compare categories and establish group membership.

For the scientist, the point here is to be aware of the degree of clarity of the categories under investigation (along with the ambiguity of metaphor that is pervasive in both everyday and scientific discourse).

Focusing

The mind not only fragments experience into meaningful categories, it also focuses our attention on a restricted range of these categories so that, at any given time, our attention consists of a foreground – the focus of attention – and a background of available categories of which we are not immediately aware (intuitively ignored).

Focus restricts the mind to a manageable number of categories at any given time. Plant classification restricts our attention to plants and the physical characteristics that comprise the elements of grouping criteria.

Grouping

Grouping[19] is the ordering of categories into groups using selection criteria that are determined by the purpose or reason for the classification.

Clearly the ordering of the objects of our experience entails the mental capacity to apply all the principles of logic; but here we will be concerned with just a few of the simple structures that are used to frame public classification systems.

Classifications provide us humans with the vital capacity to investigate the relationships between categories. With the function (purpose) of the classification known and the grouping categories established it then remains to arrange the groups into a structure or form of relations according to selection criteria, and thus producing a classification system.

Selection criteria

It is selection criteria that are used to discriminate or define the groups within the classification.

The best classifications are those that serve their purpose in the most efficient way. To maximize the efficiency of a classification we refine, as far as possible, the selection criteria that filter the objects being classified. Careful choice of selection criteria reduces the number of possible outcomes and increases the precision of the result.

Selection criterion may be simple – say, ‘being yellow-coloured’, but sometimes more complicated and amenable to improvement, as when the element ‘gold’ is selected using its atomic number rather than the properties of being, for example, a yellow, malleable, and expensive luxury good.

Structure

We have learned about the properties of the most common classification structures through the work of people like scientists studying the biological classification of organisms, and librarians working on the organization of their library catalogues, also linguists, and others.

This is a largely ignored aspect of knowledge accumulation and organization.

The structure of classifications – the arrangement of groups (as a set of compounding categories) – can take many forms and it can gather complexity with increasing numbers of categories and selection criteria.

A simple alphabetical list is a classification based on one selection criterion (letter of the alphabet). In contrast, living organisms are scientifically classified using selection criteria in such large numbers that, today, a computer is required to produce a result.

The goal of these classifications, like that of all science, is to maximize representational power. It is important to be aware of the strengths and weaknesses of these forms of classification because when we start out on a new project, wishing for clarity in our categories and their relations, a proven methodology becomes a useful asset.

Each method has its own goals, structural properties, strengths, and weaknesses.

Hierarchies

Most classification systems exhibit some kind of hierarchy, that is, they rank or prioritize their objects so that related objects are arranged in metaphorical levels (ranks), like the rungs of a ladder that has a ‘top’ and a ‘bottom’. The preponderance of classifications with some form of ranking reflects the intentional (value-driven) disposition of living organisms. And, as we shall see, this may also reflect the way that language is itself constructed.

Nested hierarchies
In a strict nested hierarchy the groups are contained within other groups – like a Russian doll which contains another doll inside, and another doll inside that one, and so on.

A scientific example would be the Linnaean biological classification of organisms into orders, which contain families, which contain genera, which contain species . . . etc. Knowing that a plant has the species name Lactuca sativa (Lettuce), connects us to a vast store of associated historical and biological information. An example of how this kind of plant classification works at high levels would be the single category ‘seed plants’ which is then divided into the groups ‘flower-bearing plants’ and ‘cone-bearing plants’. The flower-bearing plants can then be divided into those which have one seed leaf (monocotyledons) and those which have two seed leaves (dicotyledons) . . . and so on down the hierarchy of ranks.

Strict nested hierarchies like this exhibit several important properties:

Inclusivity – they are progressively more inclusive as ranks go from bottom to top
Exclusivity – an item in a strict hierarchy can only belong to one group at a particular level or rank.
Transitivity – the properties that define the objects at higher ranks are passed on to the lower ranks
Clear boundaries – the properties defining group membership at a particular rank must be both necessary and sufficient (a classical category)

An example of transitivity would be that all vertebrate animals (Vertebrata) have a vertebral column no matter how they are subsequently subdivided. Sub-groups, though resembling one-another by sharing the properties of ‘higher’ more inclusive groups, nevertheless differ in the properties that uniquely define them.

A name, as a binomial, can be used as a device to express both similarity and difference. So, Homo sapiens as a species shares the property of being human (expressed in the genus name Homo), but it differs from other species of human by being uniquely H. sapiens, the ‘wise human’. There is thus a clever association and distinction – the binomial (a name consisting of two words) expressing both similarity and difference at the same time.

An example of exclusivity would be that Homo sapiens cannot be, at the same time, Homo heidelbergensis.

Strict nested hierarchies require clarity about similarities and differences between groups. Hierarchical definitions express both affinity and distinction (similarity and difference) in an economical way.

Transitivity allows the hierarchical structure to accumulate information that is useful for prediction: inclusivity (the containment of groups within groups) demonstrates the evolutionary principle of descent with modification.

The formal structure of nested hierarchies works well for items that can be clearly defined, because they have unambiguous group boundaries. However, in daily life we use many (fuzzy) categories with indistinct boundaries so the demand for mutual exclusivity cannot be satisfactorily met.[6] In such cases the members of groups at a particular rank share a family resemblance rather than fulfilling strict necessary and sufficient conditions – and some items at a particular rank may share closer resemblance than others.

In addition, limited knowledge can diminish the power and reliability of definitions and grouping items using more than one criterion can become unwieldy (say classifying plants based on both flower structure and fruit structure).

The effectiveness of a hierarchy in achieving its purpose will only be as good as the knowledge used in its construction and lack of clarity about items and their definition diminishes the power of transitivity.

Difficulties in creating clear categories in science are well known, for example, in the classification of rapidly-changing viruses or in defining the nature of particular smells. Descent with modification can itself lead to graded transitions rather than necessary and sufficient groupings (essentialism) as we try to impose logical order on graded nature.

Modern plant classification (cladistics, phylogenetics, and molecular systematics) is based on presumed evolutionary relationships, not an arbitrary assignment to ranks; it recognizes species but does not use the terms genus, family, order, class, phylum or kingdom. Thus, cladistics has no ranks – but it does have a hierarchy which arises from the fact that some organisms share a more recent common ancestor than others as a consequence of descent with modification. A taxon that includes an ancestor and all of its descendants is called a clade.

Trees
Another kind of hierarchy is exemplified by military rank. This hierarchy has clear distinctions between groups (like private, sergeant and lieutenant) but there is minimal transitivity between the groups so, for example, the characteristics shared by a private and a sergeant are somewhat obscure.

Further, trees lack inclusivity: though a species is a member of a genus which is a member of a family – a private is not a subdivision or kind of Sergeant who is, in turn, a kind of Lieutenant. A tree indicates a chain of command but does not include a clear definition of the nature of the authority.

Tree – a nested hierarchy of Army Officers.
Trees lack inclusivity

Courtesy Wikimedia Commons – Totobaggins – Accessed 5 January 2021

Partitive hierarchies
A further kind of hierarchy is the partitive hierarchy with the part relating to a greater whole in some way. A partitive hierarchy might arise, for example, when we subdivide a geographic region. Melbourne is a part of the state of Victoria, which is a part of Australia. This is an inclusive hierarchy (though Victoria is not a kind of Australia it is a part of it) where certain properties are shared.

Partitive hierarchies share, with nested hierarchies, the characteristic of passing from the general (the most inclusive domain) to progressively smaller or less inclusive parts. But note that a simple confusion can arise here. Objects that share the characteristic of just being a part of something can be quite different in general character: an apple core, a train ticket and bottle may all be parts of the waste bin while, in a nested hierarchy of animals we know that those animals that are vertebrates will share certain similarities. Objects that appear on my weekend shopping list might have almost no connection.

Trees require a knowledge of the characteristics of the items being classified. The structure of the tree will be determined by the nature of the relationship between the parts: is it part-whole; cause-effect; process-product; start-end etc.

Trees organize categories in a way that defines how they are related and/or the degree to which they are related (spatially, metaphorically), and/or the relative frequency of items within a particular category. However, a tree is constrained by the order in which distinctions are drawn and this may require subsequent modification.

In a hierarchy the information flows (metaphorically) not only ‘upwards’ and ‘downwards’ between levels, but ‘laterally’ between items at the same level.

In trees the ‘lateral’ categories might contain very different objects so they tend to be strong along only one dimension of interest and are not so effective at representing multidirectional complex relationships. As with hierarchies the choice of key defining characteristics can be a matter of dispute and trees allow only partial inference.

Ranking

Classification may be based indifferently on the similarities and differences of its constituent categories – as, when walking around a market, I simply note fruits, vegetables, second-hand books etc. as components of my mental landscape.

However, we are intentional animals and so the categories in the foreground or focus of our attention are objects of value. That is, they are (or need to be) ranked, prioritized, or specially selected in some way depending on our needs, desires, purposes, and reasons – as when I am shopping specifically for strawberries and a cauliflower.

The ranking or prioritizing of objects relative to one-another I call rank value. The application of rank value to objects of our experience is what makes our (and other animals’) behaviour both purposive and meaningful: part of our behavioural adaptation to circumstance.

A classification of classification systems

We now have the tools to devise a classification of classification systems – by declaring its purpose, focusing on the categories to be classified, the structure of the classification, and the ranking of its products.

First, Our purpose or reason for doing this is to 

Clearly, we wish to distinguish between, say, trivial private classifications, like the grouping of objects that make up my weekend shopping list, and important public classifications like the scientific classification of all living organisms, transport timetables, or point-of-sale systems, and so on. The latter are sometimes referred to as special-purpose classifications and they require expert development using the latest technology, pushing our collective learning methodology to its limits.

Though a shopping list is important for my personal survival, it is scientific classifications that are of more general interest and therefore included in the public education curriculum.

It is worth outlining the properties of scientific classifications because we might want to devise new ones.

Scientific classification

Most scientists regard themselves as working with objects in the world, not categories of the mind, and one major purpose of scientific classification is to provide us with the best possible objective explanations (including classifications as representations) of the world as it exists outside our minds.

But there is an immediate difficulty. What categories do we use to slice up everything that there is? If all our classifications serve a purpose then what selection criteria should we use to answer this question? (see the article on worldviews and reality for some attempted answers). This is a metaphysical question to which there is no definitive answer. Does the universe ultimately consist of one thing or many? You may argue as you wish. Today we do not get an answer to this question from either the universe or God. When we ask this particular question, we do not confront the universe, we confront ourselves and our biological/intellectual limitations. There is no unequivocal answer that can be subjected to empirical investigation.[15]

Our best answer is to understand the world using the categories of science as our best possible representations. These are categories that have been tested by the use of experiment, observation, and our use of reason.  The result has been countless classifications resulting from the investigation of the world from many perspectives to provide an account of its . . . chemical elements, subatomic particles, living organisms, geological structures, and so on. Scientific classifications are our best attempts to describe the many aspects of the physical universe in a way that is open to further investigation and endless refinement.

Classifications may themselves contribute to knowledge generation. This occurred, for example, with the prediction of new and, as yet, undiscovered elements and their properties following the construction of the Periodic Table. Science was also facilitated by classifications resulting from new scientific technology, as with: carbon dating, DNA analysis, remote sensing, radio-astronomy, the crystalline structure of gems (rather than their hardness) and so on. Understanding the chemical structure of biological compounds helps establish what unknown compounds are possible but as yet undiscovered, or which may be synthesized in the laboratory . . . and so on.

The point here is that for science to proceed it must have some framework of ideas concerning the nature of ‘everything’ from which to launch its investigations. But, as explained, we have no such foundation. So, what do we do?

When we cannot resort to evidence, we fall back on our intuitions (our non-empirical assumptions) . . . and when science is asked ‘unanswerable’ (metaphysical) questions, it too falls back on intuitions.[20]

The point, if it needs to be made, is this. Although we humans are extraordinarily sophisticated in our explanation, understanding, and manipulation of matter, there will always be a residue of species-specific interpretation. It is we who provide the selection criteria for all classifications.[16]The philosophical problem of category choice for our categorizations-classification dissolves when we realize that they are decision-making tools. They impose order by organizing categories into groups using selection criteria that achieve an objective, purpose, or use.

Language

To share the categories (concepts) we form in our minds, we must communicate using spoken, written, printed, or electronic language. In this way we can refine our shared categorization which then becomes part of our potentially progressive collective learning.

Language & representation

Language provides us with a symbolic representation of the world. Cognitive scientist Steven Pinker gives an insightful example of the relationship between science and language in his book ‘The Stuff of Thought’ (2008) which shows how we embed, in a metaphorical way, the key scientific concepts of space, time, matter, and causality in everyday language. Nouns express matter as stuff 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.). 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. He remarks . . . ‘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.’

We are inclined to think of the scientific description of the world as ‘reality’ – the fixed and final way the world actually is. But science is constantly testing these beliefs. We must always remember that factual statements, even those of science, are statements of strongly corroborated evidence, not the way the world is . . . even though, in practical terms, they provide us with extremely useful explanations.

The concepts and categories that make up our language are linked into a web of interconnected knowledge. It is the task of science to organize this knowledge as the best-for-purpose that we can possibly produce.[14]

So, how are inner experiences transformed into a language that can convey information from one head to another?

Cognitive scientists and linguists have discovered that we do not think in words, we think in ‘mentalese‘.[9] To share our cogitations with others we must translate mentalese into a spoken and written language.

The way we structure language provides us with an insight into the mental process of classification itself because it entails the organization/classification of words.

Web, string, tree

We may think of the way we structure language using a metaphor proposed by cognitive scientist and linguist Steven Pinker who refers to it as ‘the web, the string, and the tree’.

This too is a miraculous process that we all take for granted. Consider the librarian’s dilemma of cataloguing the web of human knowledge into a hierarchy of linearly-related topics knowing that so many topics cut across one-another.

We have in our minds a web of interrelated categories. We organize these categories into a linear sequence (spoken or written) – a string – according to the rules of syntax which, together with the rules of word formation comprise its grammar. Grammar is our (species-specific) solution to getting complicated thoughts from one head to another. Words are arranged into phrases that are nested within sentences in the form of an inverted hierarchical tree.

‘Syntax then is an app that uses a tree of phrases to translate a web of thoughts into a string of words.’ The listener then works backwards, fitting them into a tree and recovering the links between the associated concepts.’

The hierarchy

So, we structure language like a nested hierarchy that is constructed in a boxes-within-boxes way with sentences containing substructures (phrases) that form an inverted tree when presented pictorially on the page.

Parse Tree as a Nested Hierarchy

Parse Tree as a Nested Hierarchy

S for sentence, the top-level structure in this example
NP – noun phrase. The first (leftmost) NP, a single noun “John”, is the subject of the sentence. The second one is the object of the sentence.
VP – verb phrase, which serves as the predicate
V – verb. In this case, it’s a transitive verb hit.
D – determiner, in this instance the definite article “the”
N – noun

Courtesy Wikimedia Commons – Tjo3ya – Accessed 3 January 2021

Commentary

Knowledge grows cumulatively and to make an advance in any field of study a student must add to the fine detail of their own discipline by doing classification as part of their original research . . . by refining the categories already used by their discipline, or by adding new ones.

Classification, in its most general sense, is an interaction between an organism and its environment – a process of discrimination that guides flexible behaviour. It is an ‘assessment’ of the relationships that exist between an organism and the objects on which its existence depends, its umwelt.

Human classifications have their origins in the pre-conscious structuring of the objects of our experience (categories) that we, both intuitively and deliberately, arrange in different ways to some end or reason, some ‘interest’. This is the way that we intuitively relate to the world: it is the precursor to public classifications like those of science which are the result of a formal social process of collaborative refinement that contribute to collective learning as the growth of knowledge.

During the human process of classification, categories are subjected to a mental filtering using selection criteria that serve purpose of the classification. Four necessary and interrelated aspects of this innate mental processing can be usefully distinguished: categorization, focus, grouping, and ranking.

Collectively these processes help establish our human umwelt,[1] what is important in our lives – the way we experience the world –  our sense of reality. As a way of ordering our experience classification is a poorly comprehended aspect of our faculty of reason.

Phylogenetic phenomena display the unusual characteristic of modification from a common origin are a major opportunity to apply special principles of classification.  The evolution of organisms and language are examples of the way phylogenetic (tree) analysis that can give us valuable insights into the past and can be extended to other cultural phylogenies to take full account of vertical transmission, horizontal diffusion, and local socio-ecological drivers.[18]

The article on plant classification considers the possibility of a scientific classification of plants, not in relation to other plants, but in relation to humans – a topic of greater significance for the future of humanity.

Key points

  • Our minds are constantly – both consciously and unconsciously – selecting, ordering, and arranging the objects of our experience
  • The foundations of classification are laid with the pre-conscious structuring of experience that occurs before conscious deliberation
  • Our intuitive ordering of the world is part of the way we represent the world, and it is the source of some of our metaphysical intuitions about the world. It is therefore part of our human nature and human reality, our uniquely human collective mental umwelt.
  • Included in the pre-conscious mental processing there is: segregation, focus, categorization, and ranking. These aspects of our mental processing are hard-wired into our biological make-up; without them we could not survive
  • For simplicity we can call all communicable units of experience categories, and it is these categories that can be variously grouped and prioritized according to selection criteria.
  • The grouping of units of our experience towards some end or goal may be called categorization, and when categorization becomes a serious conscious and deliberate endeavour, as it does in science, we call it ‘classification’.
  • Classification is the ordering or grouping of objects by criteria selected to achieve an end or goal.
  • Taxonomy is the study of classification, its principles, and procedures.
  • We communicate with one-another using language which provides us with a symbolic representation of the world. The categories that make up our language are linked into a web of interconnected knowledge.
  • One of the tasks of science is to ensure that the categories we use to describe and organize our collective learning are the best possible. This is a task for science.
  • Classifications are decision-making tools. There are an infinite number of ways of classifying categories but the categories chosen will depend on the purpose of the classification.
  • Biological classification of organisms arranges them by their presumed evolutionary relationships
  • One method of scientific advancement is the designation of an are of study (an academic discipline) with a community of scientists that constantly refine the categories, principles and procedures used by that study
  • Plant classification is the arrangement of plants into groups according to selection criteria that serve a particular purpose. However, in common usage, the expression ‘plant classification’ refers to the organization of plants into groups that reflect their evolutionary relationships as plant taxa which, prior to Darwin and the theory of evolution, involved the organization of mostly morphological characters as similarities and differences.
  • ‘Categorization’ is a general-purpose term. When categorization becomes a more serious and shared social activity we call it ‘classification’
  • Classification is the ordering or grouping of objects by criteria that satisfy an end or goal.
  • Taxonomy is the study of classification, its principles, and procedures.
  • Classification begins with the pre-conscious structuring of experience that occurs before conscious deliberation. This intuitive ordering of the world is part of the way we represent the world, and it is the source of some of our metaphysical intuitions about the world. It is therefore part of our human nature and human reality, our uniquely human collective mental umwelt.
  • Included in the pre-conscious mental processing that is part of our human nature are: segregation, focus, classification, and ranking. These aspects of our mental processing are hard-wired into our biological make-up; without them we could not survive
  • For simplicity we can call all communicable units of experience categories, and it is these categories that can be variously grouped and prioritized according to selection criteria.
  • We communicate with one-another using language. Language provides us with a symbolic representation of the world. The categories that make up our language are linked into a web of interconnected knowledge. We need to ensure that the categories we use to organize this knowledge are the best possible. This is a task for science.
  • Classifications are decision-making tools. There are an infinite number of ways of classifying categories but the categories chosen will depend on the purpose of the classification.
  • Biological classification of organisms arranges them by their presumed evolutionary relationships
  • One method of scientific advancement is the designation of an are of study (an academic discipline) with a community of scientists that constantly refine the categories, principles and procedures used by that study
  • Plant classification is the arrangement of plants into groups according to selection criteria that serve a particular purpose. However, in common usage, the expression ‘plant classification’ refers to the organization of plants into groups that reflect their evolutionary relationships as plant taxa which, prior to Darwin and the theory of evolution, involved the organization of mostly morphological characters as similarities and differences.
  • We study plants through the broad range of scientific disciplines that fall under the heading of plant science (formerly the narrower discipline of botany). There is no current and widely accepted system of categories, principles and procedures for the study of the topic ‘plants and people’

Epilogue

Our minds are constantly – both consciously and unconsciously – organizing their contents (categories) in a purposeful way. This ordering of categories is part of our inherited human nature, and it provides us with a coherent and meaningful experience that includes a practical representation of the world.

When this becomes a deliberate conscious endeavour, as it does in science, we describe it using the formal term ‘classification’, this being a public process undertaken as a collaborative and progressive refinement of the relationship between categories within a particular discipline as just one facet of our collective learning.

There are an infinite number of ways of classifying categories, but the categories chosen in any particular situation will depend on the purpose of the classification. So, for example, the biological classification of organisms arranges them according to their presumed evolutionary relationships. Classifications are, in this sense, decision-making tools.

Though classification, in a general sense, is the conscious or unconscious investigation of the relationship between the objects of our experience (categories or concepts). More formally, classification becomes the conscious ordering or grouping of objects by criteria selected to achieve a particular end or goal.  The study of classification itself – its principles, and procedures – is called taxonomy.

Categories are abstract entities usually denoted using names: they vary in degree of precision and clarity. A useful distinction may be made between classical categories whose defining features (selection criteria) are both necessary and sufficient, and the more common fuzzy categories whose meanings grade into the meanings of other categories. ‘Gold’, defined as an element with atomic number 79, is a classical category while ‘game’ is a fuzzy category with a family resemblance of characteristics that merges into other categories.

To be effective, our inherited mental processing must, of necessity, include at least four pre-conscious processes. The categorization of the world into meaningful units of representation (mental categories); a focus on a limited range of these objects at any given time (so that our minds operate with an experiential background and foreground); a classification of categories into various groups; and the ranking of categories and category groups in relation to one-another according to conscious and unconscious priorities.

Classifications are part of our reasoning faculty, not because they impose logic into our thinking, but because they introduce order by maximizing representational power. They arrange the categories of our experience into groups by using selection criteria that satisfy an objective or purpose and thus reflect the intentionality of biological existence. So, for example, I might wish to classify my friends using the selection criteria of hair colour, height, or sporting interests.

Every classification has three major components: its purpose (the reason why the classification was devised); the selection criteria used to arrange or group categories according to that purpose; and the structure or system of classification to be used. Systems of classification include hierarchies (see also language below), trees, lists etc. and it is important to be aware of the strengths and weaknesses of each system because, at the outset of a new project, we need clarity in our categories and their relations, so a proven methodology becomes a valuable asset.

We communicate our understanding of category relationships using the medium of language. We have in our minds a web of interrelated categories. We organize these categories into a linear sequence of words (spoken or written) – a string – according to the rules of syntax which, together with the rules of word formation comprise its grammar. Grammar is our (species-specific) solution for getting complicated thoughts from one head to another.

Words are arranged into phrases that are nested within sentences in the form of an inverted hierarchical tree using the principles of syntax. Thus, syntax uses a tree of phrases to translate a web of thoughts into a string of words. The listener then works backwards, fitting these words into a tree and recovering the links between the associated concepts. In other words, we structure language like a nested hierarchy in a boxes-within-boxes way with sentences containing substructures (phrases) that form an inverted tree when presented pictorially on the page.

There may be a connection between our intuitive hierarchical organization of language and our intuitive structuring of the world.

Language provides us with a symbolic representation of the world, its categories linked into a web of interconnected knowledge. One major task of science is to provide us with the best possible set of categories to describe and organize our collective learning.

Science has advanced by designating topics of study (academic disciplines) with a community of scientists that constantly refine the categories, principles and procedures used by that study.
When considering the world of plants and classification we place major emphasis on their scientific classification whose purpose or interest is their evolutionary relationships. Indeed, this is what most of us understand by ‘plant classification’ and it entails a well-understood set of classification categories and selection criteria. But there is another classification system whose purpose is to study, not the relationship between plants, but the relationship between plants and people for which we have no clear classification categories and selection criteria. This is the subject of the article on plant classification.

Glossary

 

category – any unit of experience that can be used for mental processing; the operational unit of mental processing
categorization – the mental formation of categories as units of representation
classification – the study of categorial relationships; the ordering of categories; the arrangement of objects into groups by selection criteria determined by the purpose of the classification; a decision-making tool. Classifications vary from being private intuitive mental processes to formally derived and communally agreed systems of representation
collective learning – culturally accumulated knowledge: shared information that is passed from generation to generation. Cultural information exists as memes that may be as simple as transmitted practical facts, or as complicated as innately comprehended body language and mental tools like language and mathematics. Historically the rate of accumulation of collective knowledge has accelerated exponentially through time facilitated, in part, by increasingly sophisticated technology.
focus – the restriction of our attention to a limited range of representational units so that, at any given time, our attention is divided into a foreground and background
purpose (taxonomy) – the reason for, or goal, of a classification
nested hierarchy – the taxa and their groupings are contained within other groups in a boxes-within-boxes fashion, like a Russian doll
rank value – the ranking (valuing or prioritizing) of objects relative to one-another within a classification system.
segregation – the organization of experience into meaningful representational units, both those of cognition (concepts), and those of perception (percepts)
plant species – (classification) the most practical unit of plant representation
taxonomy – in its broadest sense, the study of relationships. While classification is the actual process of grouping categories, taxonomy is the study of the principles and procedures that allow this to occur

**—

First published on the internet – 1 March 2019

. . . revised 11 January 2021
. . . substantially revised 28 June 2022

 

Scientific classification.

A flowering plant phylogeny of 2020
including the then recognized 435 plant families

The colours of the branches and the outer circles represent the major angiosperm clades (branches of the plant evolutionary tree) as indicated in the caption (ANA, Amborellales + Nymphaeales + Austrobaileyales). The illustrations around the circumference show 32 plant families and their position in the tree.

The flowering plants (angiosperms) arose in the Early Cretaceous about 145–100 million years ago, supplanting the earlier ferns and conifers (pteridosperms and gymnosperms). Palaeobotanical evidence indicates that this replacement was gradual, flowering plants not becoming dominant until the Palaeocene about 66–56 million years ago. The timing, geographic sequence, and plant composition of this diversification remains uncertain. There appear to have been substantial time lags, mostly around 37–56 million years, between the origin of families (stem age) and the diversification leading to extant species (crown ages) across the entire angiosperm tree of life. Families with the shortest lags occur mostly in temperate and arid biomes compared with tropical biomes. The ecological expansion of existing flowering plants, it seems, occurred long after their phylogenetic diversity originated during the Cretaceous Terrestrial Revolution.

Courtesy public domain license. Adapted from the originals provided by S.M., the Peter H. Raven Library/Missouri Botanical Garden and the Mertz Library/New York Botanical Garden

Flowering Plant Phylogeny diagram
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