PARTS OF ANIMALS, I. i.-ii. 



and turned to the practical subject of " goodness," ** 

 and to political science.) 



^ Here is an example of the method of exposition. 

 We point out that although Respiration takes place 

 for such and such a purpose, any one stage of the 

 process follows upon the others hy necessity. Neces- 

 sity means sometimes {a) that if this or that is to 

 be the final Cause and purpose, then such and such 

 things must be so ; but sometimes it means (6) that 

 things are as they are owing to their very nature, as 

 the following shows : It is necessary that the hot 

 substance should go out and come in again as it 

 offers resistance, and that the air should flow in — 

 that is obviously necessary. And the hot substance 

 within, as the cooling is produced, offers resistance, 

 and this brings about the entrance of the air from 

 \vithout and also its exit. This example shows how 

 the method works and also illustrates the sort of 

 things whose causes we have to discover. 



II. Now some ^^Titers ^ endeavour to arrive at the 

 ultimate and particular species by the process of 

 dividing the group (genus) into two differentiae.^ 

 This is a method which is in some respects difficult 

 and in other respects impossible. For example : 



the method of dichotomy is used in the Sophist and Politicus. 

 But the method can hardly be said to be seriously applied 

 to the classification of animals in the Politicus, and in the 

 Sophist it is introduced partly in a humorous way, partly 

 to lead up to the explanation of to /xt) ov (not-being). Either 

 Aristotle has mistaken the purpose of the method (as he 

 has at An. Pr. 46 a 31 if.) or (much more probably) he is 

 referring to some other writer's detailed application of it. 

 See e.g. Stenzel in Pauly-Wissowa, s.v. Speusippus. 



^ Each stage of the division gives two differentiae, which 

 are treated as " genera " for the next stage of the division, 

 and so on. 



79 



