Categories, in philosophy, the highest classes under which objects of knowledge can be systematically arranged, understood as an attempt at a comprehensive classification of all that exists. The name has come down to us from Aristotle, in whose system the categories are ten in number: Substance, quantity, quality, relation, place, time, situation, possession, action, and suffering. From the point of view of logic, these may be reduced to two: substance and attribute; of metaphysics, to being and accident. The Cartesians had the three categories—substance, attribute, and mode; Leibnitz—substance, quantity, quality, action or passion, and relation; and Locke—substance, mode, and relation. J. S. Mill classifies all existences or describable things as follows: (1) Feelings, or states of consciousness, the most comprehensive experience that the human mind can attain to, since even the external world is only known as conceived by our minds; (2) the minds which experience those feelings; (3) the bodies, or external objects, which are supposed to excite all that class of feelings that we denominate sensations; (4) the successions and co-existences, the likenesses and unlikenesses, between feelings or states of consciousness. Although those relations are considered by us to subsist between the bodies, or things, external to our minds, we are driven in the last resort to consider them as really subsisting between the states of each one's own individual mind.
The categories of Kant are conceived under a totally different point of view. The Root-notions of the understanding (Stammbegriffe des Verstandes), they are the specific forms of the a priori or formal element in rational cognition—forms inherent in the understanding, under which the mind embraces the objects of actual experience. The Kantian philosophy supposes that human knowledge is partly made up of the sensations of outward things—colour, sound, touch—and partly of mental elements or functions existing prior to all experience of the actual world. (This is the point of difference between the school of Locke, who rejected all innate ideas, conceptions, or forms, and the school of Kant. No such question was raised under the Aristotelian categories.) Kant's categories are as follows: (1) Quantity, including unity, multitude, totality; (2) Quality, including reality, negation, limitation; (3) Relation, including substance and accident, cause and effect, action and reaction; (4) Modality, which includes possibility, existence, necessity. These indicate the elements of our knowledge a priori; and though they are the necessary conditions under which alone experiences can be realised to the mind, are merely subjective forms of its own activity, distinct from and inapplicable to the world of noumena—the thing in itself—that lies outside and beyond. Fichte based the whole system of the categories of reality on the affirmation of itself by the Ego—the primitive function of self-consciousness. Hegel carried this further, and showed that this primitive function supplied the principle needed to harmonise and unify the objective and subjective elements in thought. Thought and being are ultimately identical, and the categories are thus merely definite aspects or determinations (Bestimmungen) of the universal of thought, which is identical with reality or actual existence.