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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

Our minds are constantly – both consciously and unconsciously – discriminating, filtering, comparing and ranking the objects of our experience. It is this mental classification that helps us make sense of the world.

This article discusses classification in a non-discipline-specific sense:[10] it examines the foundations of all classification systems and how they relate to scientific classifications – including their principles of construction and different structural forms. A follow-up article will then looks at the special case of scientific plant classification, including the way we classify the relationship of plants to humans and the relationship of plants to their wider environment.

Understanding the foundations of classification helps us to make a more a informed assessment of the service provided by our current scientific classifications.

Classification

Classification begins as an intuitive process in our minds, the constant mental computation that organizes, and makes meaningful, the concepts and percepts that arise in our consciousness. But classification can also be a conscious and formally organized social activity that is shared through the medium of language as part of the conscious and deliberate organization of knowledge within the cumulative social process of collective learning. Scientific classification systems are an excellent example of progressively refined collective learning.[13]

For convenience, we can call the operational units of our mental processing categories. Categorization is then the formation of mental units of representation as categories, while classification is a convenient term for the ordering of these mental categories as units of experience during mental processing. Taxonomy is, in effect, meta-classification: it is the study of the principles and procedures of classification.

Mental categories are combined in various ways to become further categories in a complex web of variously related groups and sub-groups, parts and wholes, all available for further mental processing and classification.

Viewed in this very general way, classification and taxonomy together become the study of the relationships between the objects of our experience (categories).

Mind

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 he considered the mind contributed to our understanding of the world . . . our experience of everything within the context of space and time (his transcendental aesthetic) and within certain 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.

At least four aspects of our mental processing can be usefully distinguished: 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; it classifies these categories into various groups; 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, classification, and ranking are hard-wired in us (with many others) as part of our biological make-up. We know this because they occur in all cultures and are, indeed, necessary for our survival.

So, these processes seem to occur simultaneously and with varying degrees of consciousness: this is a characterization only, drawing attention to key aspects of complex mental procedure. 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 much more miraculous and complex fusion of the conscious and intuitive. The solving of a mathematical problem seems to be a fully conscious process.

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 worth considering in more detail.

Categorization

Categorization structures our world into meaningful representational units of experience (mental categories), both those of cognition (concepts), and those of perception (percepts). The fact, for example, that 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 (classify) to provide representations of the world that, in the final analysis, constitute our human experience of the world and, at the same time, contributing to our individual and collective survival.

Focus

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.

Classification

Classification is the ordering or arrangement of categories into groups using selection criteria that are determined by the purpose or ‘interest’ of 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.

Rank value

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.

Language

For us to share with one-another the categories 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 a delightful 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.’

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

Aristotle once expressed the view that ‘everything that there is’ consists of ‘objects, their properties and relations’.[17] Inevitably philosophically contentious this does give us a starting point.

There is clearly a complicated relationship between the objects of our minds (categories) and the world – which will be discussed shortly but if we accept, for a moment, Aristotle’s contention, then it would seem that a good place to start an investigation of classification and taxonomy (as the study of relations) with the objects that are being related.

Units of classification

Once categories[11] are available as operational units of classifications,[12] what is it that determines the particular categories we select for use in these classifications? What determines the selection criteria used to group the categories?

Two important points arise here, one concerning the purposes of classifications and the other concerning our intuitions:

Purpose, utility, reasons

Classifications do not exist in the world, they exist as representations of the world that are in our minds, and our minds operate intentionally: they are goal-directed. Consider any grouping of categories that you might make and it is obvious that it has been established for some purpose, use, or reason – to order some property or characteristic, to assess desirability, to assist prioritization, and so on.

There are a seemingly limitless ways to classify categories, but we do not create random classifications, we construct them for a reason or purpose. This aspect of classification will, to simplify discussion, be referred to from now on as its ‘interest’ recognizing that all classifications are decision-making tools that provide answers to questions that we pose concerning relationships between the objects of our inner and outer worlds.

Mind & world

Most scientists regard themselves as working with objects in the world, not categories of the mind.

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. To do this we attempt to minimize any human bias that might infect the scientific attempt to describe the ‘true nature’ of the world. The categories of science are our best possible representations of the external physical world.

We have widened the human horizons of our common sense reality (the manifest image), by using sophisticated technology to inspect the cosmos at the micro- and macro- scales that lie beyond our unaided senses. In this we have been extraordinarily successful. But we cannot examine the universe with a God’s-eye view, or from the perspective of the universe itself. We cannot step outside our minds to see the world ‘as it really is’ (this does not make sense). No matter how objective we try to be, our view of the world is always, in this sense, our human view.

By the use of experiment, observation, and reason, science has provided countless classifications to investigate different aspects of the world . . . its chemical elements, subatomic particles, living organisms, geology, and so on. These are all our best descriptions of matter examined in diverse forms and aspects as determined by our human selection criteria.

But how is science to divide up the totality – how is it to classify everything that there is? What selection criteria should we use to answer this question? See the article on worldviews for some attempts at answers. This is a metaphysical question. There is no definitive answer to this question that has incontrovertible evidence. Does the universe ultimately consist of one thing or many? You may argue as you wish. 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] As with all classifications, our classification of ‘reality’ will depend on the ‘interest’ of the selection criteria that we use.

Even so, science is the most detached and objective account of the universe that we have. Scientific (empirical) 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.

Intuition

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

There are, at present, two competing scientific perspectives (intuitions or representations) of ‘everything that there is’ – the analytical reductionist view (that the physical world is grounded in the world’s fundamental particle-wave-fields) or the emergentist view (that the world is best explained as a system of hierarchically integrated layers ranked largely by scale, inclusiveness, and complexity). This is a view of the world that is reminiscent of the way that we structure language – and, indeed, it is possibly related to it.

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.

Names

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).

Systems of classification

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.

It is now time to look more closely at the different ways that we can group categories to create further categories. Are some better than others . . . and how do we go about assessing their relative merits?

Aristotle once expressed ‘everything’ as consisting of objects, their properties and relations. Adopting Aristotle’s perspective we can see that classifications provide us humans with the vital capacity to investigate the relationships between categories. Once these categories, as units of classification, have been established, then the classification, as a particular set of relations is constructed out of three major elements: the purpose (reason) why the classification was devised; the selection criteria used to organize categories according to that purpose; and the structure or system of classification to be used.

General & special purpose

It is worth repeating that most of our mental classification is of no interest or consequence for other people. This aspect of our mental classification can be described as ‘general purpose classification’.

But sometimes the classification of categories becomes an important social activity, examples being scientific classifications, transport timetables, point-of-sale systems, and so on. These formal classifications in the public domain often require expert development using the latest technology, pushing our collective learning methodology to its limits. These socially important public classification systems are, in this sense, special-purpose classifications.

Selection criteria

The best classifications are those that serve their interest 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 scientists (such as those working on the biological classification of organisms), librarians working on the organization of their library catalogues, linguists, and others: it is an element of knowledge organization and library science.

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.

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, like the rungs of a ladder with a ‘top’ and ‘bottom’. The preponderance of classifications with rank value occurs because humans are intentional and value-driven creatures. And, as we have seen, this is 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, which contains another doll, 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 pass 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’ 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 in an economical way.

Transitivity allows the hierarchical structure to build up information that is useful for prediction, while 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 categories with indistinct boundaries so the demand for mutual exclusivity cannot be met satisfactorily.[6] In such cases the members of groups at a particular rank sharing 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, phylogenetic and molecular systematics) is based on presumed evolutionary relationships, rather than arbitrary assignments into 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 with each other than they do with other organisms, a consequence of descent with modification. A taxon that includes an ancestor and all of its descendents 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.

In sum, 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. 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 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.

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: 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.

Relative merits

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.

Foundations of taxonomy

Knowledge grows cumulatively and to make an impact in any field of study a student must add to the fine detail of their own discipline by refining the categories that help define their discipline, or by adding new ones (doing original research).

This lengthy introduction to classification has involved many complex ideas. Before moving on to the specific instance of plant classification some conclusions may be drawn about taxonomy in general as it relates to the topics discussed on this web site.

Classification, in the most general terms, is the study of relationships – a way of establishing connections between the objects of our experience both those of our private inner world and those categorical representations of the physical and public world outside our minds. Classification has its 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 (innately) relate to the world and it is, so to say, a precursor to public classifications (like those of science) that are the result of a formal social process of collaborative refinement.

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

Collectively these processes help establish our human umwelt[1] – our sense of reality and what is important in our lives . . . the way that the world is (to us humans). As a biological phenomenon classification (which includes all the logical procedures of our rational faculty) has helped us to manage our daily lives and, ultimately, to survive, and flourish – both as individuals and communities. Classification is therefore a key aspect of the human-defining characteristic ‘reason‘.

Scientific classification provide us with the best possible objective explanations of the world outside our minds, although the human element can never be completely erased. Above all, they are progressive, telling us much about the physical world. But to address modern problems, such as that of sustainability, our knowledge of scientific taxonomy can be applied more broadly.

As students we can become enmeshed in our own region of the knowledge web. Proceeding by analysis and the pursuit of finer detail, we do indeed ‘learn more and more about less and less’. By drilling ever deeper analytically within a narrow topic it can become increasingly difficult to establish a synoptic view of the intellectual landscape – something we desperately need to reassure ourselves that effort and resources are being directed towards the areas of greatest need confronting humanity today such as environmental problems at the macro scale.

Phylogenetic phenomena, those that evolve over time (most notably organisms and language) provide an opportunity for the methods of phylogenetic (tree) analysis that can give us valuable insights into the past. These phylogenies are a useful tool for a wide range of cultural phenomena (esp. archaeology, anthropology and linguistics). Cultural phylogenies should take full account of vertical transmission, horizontal diffusion, and local socio-ecological drivers.[18]

The next article examines plant classification, not in a narrow biological sense, but in relation to 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.
    • 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’
    • ‘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’

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
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

 

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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