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At least the great run of our thoughts involve categories.
Roger Scruton says: experience is irremediably particular (Scruton, A Short History of Modern Philosophy, London, 1981, Routledge, p.90).
Though we can look at this cat or that cat, we never see the category cat. But the empiricists hold that all our knowledge must be built out of what our senses provide us with. So if we use categories in our thinking and in our developing knowledge those categories must somehow be built by us out of the raw material of particular experiences that our senses provide.
This is a challenge that empiricists must address.
Porphyry lived in the 3rd Century AD and wrote an introduction (in Greek) to Aristotle's Categories. It was called the Isagoge. The Isagoge impacted on medieval philosophy two centuries after it had been written, via a thinker called Boethius. He translated the Isagoge and offered his own commentary on it.
The question suggested and taken up by medieval philosophers was the question of the existence of categories. Did they exist, and if so in what way? Were they actual bodily things, or things but not bodily things, or perhaps they didn't really exist independently at all?
If this is the question, Plato is usually interpreted as having answered (earlier!): categories do indeed exist. Categories as he thought of them are sometimes called Forms and sometimes Ideas, and they belonged to a world of their own.
What is it that makes this animal that we see in the world of sense a horse? The answer that Plato's theory gives is that it is somehow a horse in virtue of there being an Idea of Horse in the world of Ideas, and in virtue of the animal that we see somehow being 'informed' by that.
Aristotle had disagreed.
At least you could take this from him: just as a matter of observation we can see that matter is parceled up in our world in a limited number of ways - for example there are a limited number of forms of animals out there.
Does an austere Aristotelian concept of form entail the existence of categories?
My own thought on this is that it depends on how he thinks the categories he is recognising are produced, on what he thinks brings it about that things fall into one or other of a limited number of categories.
What makes a parcel of matter a thing of a particular sort (i.e. a thing that falls under a particular category) is a sort of quasi-component which is somehow 'in' the matter.
The form is thought of also as encompassing a representation of the end state of development, and as providing the drive which initiates and then moves development forward.
If you think of each individual thing as having a form which produces its mature organisation, then your concept of a form is of a thing that exists. And if your account of categories is that forms produce them, your theory of forms involves you thinking that categories exist.
On the other hand, if you refuse to allow the austere notion of form to be carried away by the medievals and turned into a quasi-component of a thing, it is possible to retain the idea that categories do not exist. You may ask What explains the fact that there are a limited number of forms or things? But this needn't invoke the existence of categories, which the Scholastic elaboration of form in effect does.
Rejecting the state thought had got into under Scholasticism, the Modern John Locke is as aware as Aristotle had been that the things in the world appear to fall into a limited number of sorts and of faces afresh the question: what is their ontological status? These sorts that things appear to fall into - do they somehow 'exist'? Are there categories?
Locke: categories are 'the workmanship of the understanding'. My 'understanding' generates a category out of a number of ideas of individual things by identifying all the component ideas which the ideas of individual things have in common. This collection of ideas which are common to all the ideas of individual things that my understanding is considering constitute the 'essence' of the category I am in this way constructing. (He calls this its 'nominal' essence.)
So the properties of individual things were down to the properties of the particles that made them up and their configuration. And if those properties exhibited patterns - another way of putting the point that individual things appeared to fall into a limited number of categories - the patterns would be explicable in terms of the properties of their constituent particles as well.
So sorts are human constructs, but they are human constructs which reflect the real state of the world - the real properties of atoms, and the real state of their configuration.
The suggestion is that we can explain all that needs to be explained regarding categories, as distinct from individual things, by recognising a certain type of word.
Berkeley is often said to be a nominalist in this sense, and perhaps he was one. But it is difficult to work out what positive defensible doctrine he has to offer. Much more developed and trenchant is his attack on the claims about generality he thought he had identified in Locke.
Aaron: ''to understand universals is to begin to understand thinking'. (Aaron, TU, p.vii)
Maybe the question is: how can we program a machine to handle categories with human sophistication?
