1. I. 3 



and the final cause is still more dominant than in works of art, 

 such as these, and necessity is a much less constant factor in their 

 production ; though it is to this that almost all writers would 

 seek to refer their origin, though they do not distinguish the 

 various senses in which the term necessity is used. For there is 

 absolute necessity, manifested in eternal phenomena ; and there 

 is hypothetical necessity, manifested in everything that is 

 generated by nature as in everything that is produced by art, 

 be it a house or what it may. For if a house or other such 

 final object is to be realised, it is necessary that such and such 

 material shall exist ; and it is necessary that first this and then 

 that shall be produced, and first this and then that set in motion, 

 and so on in continuous succession, until the end and final result 

 is reached, for the sake of which each prior thing is produced and 

 exists. As with these productions of art, so also is it with the 

 productions of nature. The mode of necessity, however, and the 

 mode of ratiocination are different in natural science [and in art] 

 from what they are in the theoretical sciences ;* of which we have 

 spoken elsewhere. For in the latter the starting-point is that 

 which is ; in the former that which is to be. For it is that which 

 is yet to be — health, let us say, or a man — which, owing to its 

 being of such and such characters, necessitates the pre-existence 

 or previous production of this and that antecedent ; and not this 

 or that antecedent which, because it exists or is generated, makes 

 it necessary that health or a man shall come into existence. Nor 

 is it possible to trace back the series of necessary antecedents to 

 a starting-point, of which you can say that, existing itself from 

 eternity, it has determined their existence as its consequent. 

 These, however, again are matters that have been dealt with in 

 another treatise. There too it was stated in what cases absolute 

 and hypothetical necessity exist ; in what cases also the propo- 

 sition expressing hypothetical necessity is simply convertible, and 

 what cause it is that determines this convertibility.^ 



Another matter which must not be passed over without con- 

 sideration is, whether the proper subject of our exposition is that 

 with which the ancient writers concerned themselves, namely, what 

 is the process of formation of each animal; or whether it is not 

 rather, what are the characters of a given creature when formed. 

 For there is no small difference between these two views.^ The 

 best course appears to be that we should follow the method 

 640 a. 



