The reform of diairesis

Aristotle's comments on logical division (diairesis) evidently refer to a more regular method than the examples given in Plato's late dialogues, but unfortunately no reliable evidence survives of Academic practice between the dialogues and Aristotle's Topics. We cannot know how much of the rules that Aristotle lays down may represent a codification of existing methods, and how much is correction of them. It is clear, however, that the criticisms and rules, which he sets out with great profusion of detail in the Topics and Analytics, are intended to ensure a good method, not merely to demolish bad ones, so that the picture which they build up may be taken as the form of diairesis of which he himself approved. It is very different from that set out by Plato, and the difference is exactly what we should expect after the change in philosophical viewpoint from Plato's theory of forms to Aristotle's theory of the substantial tode ti. Aristotle presents three major innovations: (i) the ontological distinctions between genus, differentia, species, property, essential and inessential accident, and other formal categories, which Plato did not distinguish; (ii) the insistence on successive differentiation, to preserve the unity of definition; (iii) division by a plurality of differentiae simultaneously, instead of by one at a time. These improvements are made respectively in (i) Topics and Categories; (ii) Posterior Analytics and Metaphysics; (iii) De Partibus Animalium 1.