10 1. I — i. 2. 



first who came near it was Democritus, and he was far from 

 adopting- it as a necessary method in natural science, but was 

 merely brought to it, spite of himself, by constraint of facts. In 

 the time of Socrates a nearer approach was made to the method. 

 But at this period men gave up enquiring into the works of 

 nature, and philosophers diverted their attention to political 

 science and to the virtues which benefit mankind. 



Of the method itself the following is an example. In dealing 

 with respiration we must show 'that it takes place for such or 

 such a final object ; and we must also show that this and that 

 part of the process is necessitated by this and that other stage 

 of it. By necessity we shall sometimes mean hypothetical neces- 

 sity, the necessity, that is, that the requisite antecedents shall be 

 there, if the final end is to be reached ; and sometimes abso- 

 lute necessity, such necessity as that between substances and their 

 inherent properties and characters. Thus it is necessary [if we 

 are to live] that there shall be alternating discharges and returns 

 of heat from and to the body, and a necessary condition for this 

 is the inflow of air. Here at once we have a necessity [in the 

 former of the two senses]. But the reduction of the internal heat by 

 refrigeration produces [as a necessary result] the outflow of the 

 air, while vice versd the reduction of the cold by the internal 

 heat produces an inflow. [This is a necessity in the second of 

 the two senses.] ^^ 



In the foregoing we have an example of the method which we 

 must adopt, and also an example of the kind of phenomena, the 

 causes of which we have to investigate. 



(Ch. 2.) Some ^ writers propose to reach the definitions of the 

 ultimate forms of animal life by bipartite division. But this 

 method is often difficult, and often impracticable. 



Sometimes the final differentia of the subdivision is sufficient 

 by itself and the antecedent differentiae are mere surplusage. 

 Thus in the series Footed, Two-footed, Cloven-footed,^ the last 

 term is all-expressive by itself, and to append the higher terms 

 is only an idle iteration. 



Again it is not permissible to break up a natural group, Birds 

 for instance, by putting its members under different bifurcations, 

 as is done in the published dichotomies, where some birds are 

 ranked with Animals of the water, and others with Animals of the 

 air. The group Birds and the group Fishes happen to be named, 

 642b. 



