1. I. 



same seed then is the seed both of the horse and of the mule, 

 though in different ways. Moreover, the seed is potentially that 

 which will spring from it, and the relation of potentiality to 

 actuality we know [to be that of consequent to antecedent].^^ 



There are then two causes, namely, necessity and the final end. 

 For many things are produced, simply as the results of necessity. 

 It may, however, be asked, of what mode of necessity ^* are we 

 speaking when we say this. For it can be of neither of those 

 two modes which are set forth in the philosophical treatises.-'^ 

 There is, however, the third mode, in such things at any rate as are 

 generated. For instance, we say that food is necessary ; because 

 an animal cannot possibly do without it. This third mode is what 

 may be called hypothetical necessity. Here is another example 

 of it. If a piece of wood is to be split with an axe, the axe must 

 of necessity be hard ; and, if hard, must of necessity be made of 

 iron, or bronze, or the like. Now exactly in the same way the 

 body, which like the axe is an instrument — for both the body 

 as a whole and its several parts individually have definite opera- 

 tions for which they are made — ^just in the same way, I say, the 

 body, if it is to do its work, must of necessity be of such and 

 such a character, and made of such and such materials. 



There are then, as before said, two modes of causation, and both 

 of these must, so far as possible, be taken into account in explain- 

 ing the works of nature.^^ At any rate it is plain that an attempt 

 must be made to include them both ; and that those who fail in 

 this tell us in reality nothing about nature. For the main factor 

 in the nature of an animal is much more the final cause than 

 the necessary material. There are indeed passages in which even 

 Empedocles hits upon this, and following the guidance of fact, 

 finds himself constrained to speak of the final cause as constituting 

 the essence and real nature of things. Such, for instance, is the 

 case when he explains what is a bone. For he does not merely 

 describe its material, and say it is this one element, or those two 

 or three elements, or a compound of all the elements, but states the 

 law or plan of their combination, and takes this to constitute the 

 bone. As with a bone, so manifestly is it with the flesh and all 

 other similar parts. 



The reason why our predecessors failed in hitting upon this 

 method of treatment was, that they were not in possession of the 

 notion of formal cause, nor of any definition of essence.^''' The 

 642 a. 



