Book 3
3
But from the point of view that it is through definitions that we get to know each particular thing, and that the genera are the first principles of definitions, the genera must also be the first principles of the things defined.
4
And if to gain scientific knowledge of things is to gain it of the species after which things are named, the genera are first principles of the species. And apparently some even of thoseThe Pythagoreans and Plato. who call Unity or Being or the Great and Small elements of things treat them as genera.
4
Nor again is it possible to speak of the first principles in both senses.
5
The formula of substance is one; but the definition by genera will be different from that which tells us of what parts a thing is composed.
5
Moreover, assuming that the genera are first principles in the truest sense, are we to consider the primary genera to be first principles, or the final terms predicated of individuals? This question too involves some dispute.
6
For if universals are always more truly first principles, clearly the answer will be the highest genera, since these are predicated of everything. Then there will be as many first principles of things as there are primary genera, and so both Unity and Being will be first principles and substances, since they are in the highest degree predicated of all things.
7
But it is impossible for either Unity or Being to be one genus of existing things. For there must be differentiae of each genus, and each differentia must be onei.e., each differentia must have Being and Unity predicated of it.; but it is impossible either for the species of the genus to be predicated of the specific differentiae, or for the genus to be predicated without its species.The reasons are given in Aristot. Topica, 144a 36-b11. Hence if Unity or Being is a genus, there will be no differentia Being or Unity.
