Metaphysics is a broad area of philosophy marked out by two types of inquiry. The first aims to be the most general investigation possible into the nature of reality: are there principles applying to everything that is real, to all that is? – if we abstract from the particular nature of existing things that which distinguishes them from each other, what can we know about them merely in virtue of the fact that they exist? The second type of inquiry seeks to uncover what is ultimately real, frequently offering answers in sharp contrast to our everyday experience of the world.

Understood in terms of these two questions, metaphysics is very closely related to ontology, which is usually taken to involve both ‘what is existence (being)?’ and ‘what (fundamentally distinct) types of thing exist? The two questions are not the same, since someone quite unworried by the possibility that the world might really be otherwise than it appears (and therefore regarding the second investigation as a completely trivial one) might still be engaged by the question of whether there were any general truths applicable to all existing things. But although different, the questions are related: one might well expect a philosopher’s answer to the first to provide at least the underpinnings of their answer to the second.

Aristotle proposed the first of these investigations. He called it ‘first philosophy’, sometimes also ‘the science of being’ (more-or-less what ‘ontology’ means); but at some point in antiquity his writings on the topic came to be known as the ‘metaphysics’ – from the Greek for ‘after natural things’, that is, what comes after the study of nature. This is as much as we know of the origin of the word (see Aristotle §11 and following). Any attempt on either question will find itself using, and investigating, the concepts of being and existence (see Being; Existence). It will then be natural to ask whether there are any further, more detailed classifications under which everything real falls, and a positive answer to this question brings us to a doctrine of categories (see Categories).

The historical picture here is complex, however. The two main exponents of such a doctrine are Aristotle and Kant. In Aristotle’s case it is unclear whether he saw it as a doctrine about things and their basic properties or about language and its basic predicates; whereas Kant quite explicitly used his categories as features of our way of thinking, and so applied them only to things as they appear to us, not as they really or ultimately are (see Kant, I.). Following on from Kant, Hegel consciously gave his categories both roles, and arranged his answer to the other metaphysical question (about the true underlying nature of reality) so as to make this possible (see Hegel, G.W.F.).