What is the philosophical study of classification called? Taxonomy? Taxology? "Category theory"? "Classificology"? Logic?

And which philosopher(s) studied exactly what it means to classify? How we classify? What classification tells us? Whether classifications are explanations of what they classify? Whether there are true and false classifications? Can we judge the goodness or badness of a classification? If so, how? etc.

I'm not looking for how philosophers have classified things (e.g., the Tree of Porphyry or Aristotle's Categories), but exactly what classification itself is.

The answer here says

The concept to classify by division has already been used - and possibly invented - by Plato under the name dihairesis (= division). Plato starts his work Sophistes with several examples of this method.

Did Plato also study classification itself?

It would seem the study of classification is part of logic, since logic is the art whereby we organize our thoughts, and this is exactly what classification does.

6 Answers 6


The study of classification at the very basic level, i.e. as studied by Aristotle, and going all the way to Kant, is part of Ontology. A topic in ontology, more specifically in metaphysics, is the study of categories of being.

There is also a field of mathematics called category theory, which through it relationship with formal logic, ends up having implications for philosophy - but more related to philosophical logic and argumentation than to ontology itself.

The wikipedia page and the SEP page, as generic as they are, are actually both pretty good places to start.

  • Ontology is the study of being, not the study of classification. I'm somewhat implicitly asking about what the relation between being and classification is.
    – Geremia
    Commented Dec 16, 2015 at 19:15
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    The fundamental categorization and sorting of things is an ontological question. For example "do mind and matter belong to the same category or different categories?" is an ontological question. You might want to reexamine your definition of ontology. Commented Dec 16, 2015 at 19:28
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    @Geremia based on your comment, it seems that what you are looking for are the concepts of clustering and classification. Strictly speaking, they are topics in mathematics and AI, not philosophy. But the results of those fields can be applied to Natural Kinds and Conceptual Clustering Commented Dec 16, 2015 at 19:45
  • thanks for the link to "Natural Kinds". That seems very relevant here.
    – Geremia
    Commented Dec 16, 2015 at 19:52

If we acept that cognitive science is a natural generalization of philosophy, then "Prototype Theory" is probably what would come close to be an answer here. What's more, it can be linked to the famous "Family resemblance" that made much for Wittgenstein's popularity.

Charles Peirce has also written at some length on classifications (especially classifications of science which is indeed a philosophical task). There is also an interesting chapter on Classification in The order of Things (written in 1966 by the younger Foucault).


Classification is studied in Knowledge Organization, an interdisciplinary branch of Library and Information Science. It started from practical questions how to actually organize abstract objects but shifted to more theoretical questions since purely practical aspects are covered by computer science, especially information retrieval and artificial intelligence.

The outcome of classification as intellectual process of grouping concepts is more broadly referred to as Knowledge Organization System (KOS). See the ISKO encyclopedia article on KOS for an overview, including references to philosophical background and references such as Peirce and Wittgenstein.


In modern logic there is no "classification" topic.

About the mathematical Category theory, at most we can say that gives us a "classification" of mathematical structures.

The term 'Taxonomy' is mostly relevant in the biological sciences.

Some philosophical aspects can be treated as subtopics to Natural Kinds (see also Species).


Scholars of C. S. Peirce have invented the term "categoriology" to describe the study of classification.

Peirce gives the following definitions of class[ification]:

1893 [c.] | Grand Logic: Book I. Of Reasoning in General. Introduction. The Association of Ideas | CP 7.392
As experience clusters certain ideas into sets, so does the mind too, by its occult nature, cluster certain ideas into sets. These sets have various forms of connection. The simplest are sets of things all on one footing and agreeing in each belonging to the set. Such a set is a class. The clustering of ideas into classes is the simplest form which the association of ideas by the occult nature of ideas, or of the mind, can take.
1898 | Cambridge Lectures on Reasoning and the Logic of Things: Detached Ideas on Vitally Important Topics. Lecture II | CP 4.5
…a class is a set of objects comprising all that stand to one another in a special relation of similarity.
1902 | Minute Logic: Of the Classification of the Sciences. Second Paper. Of the Practical Sciences | MS 1343:11
Every class is constituted and held together by a concept or idea expressed in its definition.
1902 | Minute Logic: Chapter II. Prelogical Notions. Section I. Classification of the Sciences (Logic II) | EP 2:117; CP 1.204
A class […] is the total of whatever objects there may be in the universe which are of a certain description.

He also defines "natural classification:"

1902 | Minute Logic: Of the Classification of the Sciences. Second Paper. Of the Practical Sciences | MS 1343:11-12
Every class is constituted and held together by a concept or idea expressed in its definition. Every arrangement of ideas is itself an idea. Consequently, every classification whatever is governed by an idea, however loose and incongruous it may be. A natural classification, that is to say, a birth-al classification, is a classification whose governing idea coincides with the idea which determines the things classified to exist. An idea, so far as it has any relation to life, is a possible purpose. [—] Should there be no human purpose, there may, nevertheless, be an evolutionary agency that acts like a purpose, or there may be a principle similar to such agency except that it is related, not to a temporal, but to a logical sequence of results.
1902 | Minute Logic: Chapter II. Prelogical Notions. Section I. Classification of the Sciences (Logic II) | EP 2:127; CP 1.227
All natural classification is […] essentially, we may almost say, an attempt to find out the true genesis of the objects classified. But by genesis must be understood, not the efficient action which produces the whole by producing the parts, but the final action which produces the parts because they are needed to make the whole. Genesis is production from ideas. It may be difficult to understand how this is true in the biological world, though there is proof enough that it is so. But in regard to science it is a proposition easily enough intelligible. A science is defined by its problem; and its problem is clearly formulated on the basis of abstracter science. This is all I intended to say here concerning classification, in general.
1902 | Minute Logic: Chapter II. Prelogical Notions. Section I. Classification of the Sciences (Logic II) | EP 2:128-9; CP 1.231
All classification, whether artificial or natural, is the arrangement of objects according to ideas. A natural classification is the arrangement of them according to those ideas from which their existence results. No greater merit can a taxonomist have than that of having his eyes open to the ideas in nature; no more deplorable blindness can afflict him than that of not seeing that there are ideas in nature which determine the existence of objects. The definitions of Agassiz will, at least, do us the service of directing our attention to the supreme importance of bearing in mind the final cause of objects in finding out their own natural classifications.

It is interesting he relates final causality to classification.


Classification can be taken in two ways: the simple act of dividing a subject into its main divisions, which anyone can do after memorizing a Ramean tree for the subject; or a second more comprehensive meaning which encompasses the "understanding" aspect of the subject, by defining classification not simply as a static Ramean tree, but as a process of inventing such a tree from one's own understanding of the subject.

My claim is that when taken in this second sense, classification and understanding are one and the same. If we then define logic as the art of conducting one's reason in order to attain to true knowledge (ie understanding), then classification is the same as logic.

The reason for this seems to be because classifying something requires classifying it from first principles and deducing the various divisions of the subject based on one's understanding of how the subject stems from first principles, in the same way as Euclid's elements derives geometry from Euclidean axioms and definitions. However, notice that this is the definition of understanding itself, namely, to be able to derive a subject from first principles. This is why classification and understanding are the same thing, and each implies the other: being able to classify requires understanding, and understanding implies being able to classify comprehensively (from first principles).

A practical application of this to one's own studies is that classification can be used as a kind of "barometer" for testing your understanding. The better you can classify, the better your understanding of the subject. Again, it must be emphasized that this classification should stem from one's own understanding for this barometer to work; otherwise, anyone can memorize a classification of a subject. Classification is thus a very powerful tool for measuring how well one is progressing in a subject. However, the reverse can also be applied: meditating on one's own on classifying a subject, or a small chapter of a subject, will directly lead to understanding this portion of the subject from first principles, due to the definition of classification we gave above. Thus, there is a symbiotic relationship between the two, and when both are used together we end up with a very powerful method for attaining understanding.

One can proceed simply by trying to understand, which is the non-philosophical way to learning used by most students. One could also focus instead on classification. However a mixture of the two methods seems to be best, in which case one would periodically review one's ability to classify and use this as a barometer for areas where one needs to work more on understanding. This is indeed how mindmaps function: one attempts to write out the subject from memory using mindmaps (ie Ramean trees), and where one fails is where one needs to do more work at understanding from first principles (or just understanding) - the efficient cause for classification.

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    If you have any references this would support your answer and give the reader a place to go for more information. Commented May 9, 2019 at 17:14

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