The doctrine is fairly clear. Its justification is quite obscure. Since we are concerned with the historical role of the doctrine we will consider Aristotle's justification only to the degree helpful in clarifying the doctrine. Aristotle's analysis of predication was concerned with ontological priority. Unfortunately, his analysis is notoriously innocent of any use/mention distinction, so it is not always clear when he is speaking about terms and when about the things terms signify. Further, he seems to equate meanings with definitions. Instead of beginning with such contemporary concerns, it is better to begin by situating the doctrine of the categories between Plato's late dialogs and Aristotle's later metaphysics. For Plato scientific knowledge (episteme, not modern science) must be certain and of what is. Forms, rather than changeable beings fit the requirement. Changeable beings were understood in terms of one quality replacing another, as heat replacing cold or fire either retreating or perishing in the presence of cold (Phaedo, 103). Concrete individuals were conceived of, in Sellars's apt phrase, as leaky bundles of particulars. Aristotle was using an analysis of predication to get at the reality of things. Effectively he treated concrete individuals as falling into one of three classes: heaps, natural units, and artifacts. Heaps fit the Platonic treatment of individuals as leaky bundles of particulars. Aristotle considered natural units to be objects of scientific knowledge, and in fact devoted much of his career to studying them. He constantly relies on the analogy between natural units and artifacts. A piece of leather becomes a sandal because of an imposed form and a purpose. His doctrine of forms came later. In the Categories he is concerned with getting at concrete units by analyzing predication. That aspect, however, should be situated in the context of the Socrates of Plato's dialogs unceasingly searching for the true definitions of justice, piety, and other virtues .
Aristotle's initial discussion introduces two types of distinctions in talking about terms. The first set, 'equivocal', 'univocal' and 'derivative' (e.g., gambler, runner) depends crucially on features of language. The second set, 'said of' and 'present in' do not manifest the same dependence on the terms used (and so are more concerned with things than terms). They differ in transitivity of predication. Thus (as in Greek, omitting indefinite articles) if "Socrates is man" and 'Man is animal" then "Socrates is animal" and the definitions of 'man' and 'animal' can be predicated of Socrates. Affirming "Socrates is pale", however, does not imply that the definition of pale white applies to Socrates. It applies to colors. Putting the two distinctions together yields a fourfold classification. (a) Some items can be both in something as subject, and also said of something; (b) other items are in but not said of something; some items can only be said of something as subject but are not in anything; finally some items can neither be said of anything nor be in anything. The last class yields the concrete objects Aristotle treats as basic. The other classes involve problematic features concerning predication.
To get at the distinctive features of the objects Aristotle took as basic we consider the role of definitions. A definition is a phrase signifying a thing's essence (Topics 101b38). It is an answer to the question "What is an X?" Plato thought that there could be no definition of sensible things. (Metaphysics, 987b) For Aristotle, individual things are what they are because they are beings of a certain kind. (Topics, 125b 37-39) This kind could be defined by a genus and specific difference. Their designation was through univocal terms that his logic required. Thus, the primary instances of ‘things’ are natural units.