These two chapters (642 b 5–644 a 11) form a continuous argument in which Aristotle offers three reasons for rejecting ‘dichotomy’—a misnomer, as it turns out. It is the only passage where he mentions dichotomy, and the three reasons against it do not appear in his discussions of Division elsewhere. They are:

1. 1. The final differentia of a dichotomy is inadequate to mark out the species;

2. 2. Dichotomy cannot avoid splitting up natural kinds, like Birds;

3. 3. Dichotomy cannot make use of privative differentiae (στερήσεις).

The rest of his argument is in support of these three reasons, as I will try to show. It falls into separate sections in which he adduces various rules and practices of Division to show that if Dichotomy is properly conducted according to these rules it cannot work. The rules that he gives here are all found elsewhere, with one minor exception at 643 a 35 which is applicable only to zoology. Neither here nor elsewhere does he suggest that division in general is unsound or useless. (He denies that it can prove, but allows that it helps towards finding a definition, An. Pr. 1. 31; An. Post. 11. 5 and 13.) True, dichotomy is a particular kind of division. But having shown that it is useless in zoology, he replaces it with another kind of division.