20 ii. I. 



sake of the house ; and the same is the case with the materials 

 of other bodies. For generation is a process from a something t6 

 a something ; that which is generated having a cause in which it 

 originates and a cause in which it ends. The originating cause is 

 the primary efficient cause, which is something already endowed 

 with tangible existence, while the final cause is some definite [and 

 predictable] form ; for man generates man, and plant generates 

 plant, and in each case out of the material which environs both.)^ 

 In order of time then the material and the generative process 

 must necessarily be anterior to the being that is generated ; but 

 in logical order the definitive character and form of each being 

 precedes the material. This is evident if one only tries to define 

 the process of formation. For the definition of house-building 

 includes and presupposes that of the house ; but the definition of 

 the house does not include nor presuppose that of house-building ; 

 and the same is true of all other productions. So that it must 

 necessarily be that the elementary material exists for the sake 

 of the homogeneous parts, seeing that these are genetically 

 posterior to it, just as the heterogeneous parts are posterior 

 genetically to them.'' For these heterogeneous parts have reached 

 the end and goal, having the third degree of composition, in 

 which degree generation or development often attains its final 

 term.^ 



Animals then are composed of homogeneous parts, and are also 

 composed of heterogeneous parts. The former however exist for 

 the sake of the latter.^ For the active functions and operations 

 of the body are carried on by these ; that is, by the heterogeneous 

 parts, such as the eye, the nostril, the whole face, the fingers, the 

 hand, and the whole arm. But inasmuch as there is a great 

 variety in the functions and motions not only of aggregate animals 

 but also of the individual organs, it is necessary that the sub- 

 stances out of which these are composed shall present a diversity 

 of properties. For some purposes softness is advantageous, for 

 others hardness ; some parts must be capable of extension, others 

 of flexion. Such properties then are distributed separately to the 

 different homogeneous parts, one being soft another hard, one 

 fluid another solid, one viscous another brittle ; whereas each of 

 the heterogeneous parts presents a combination of multifarious 

 properties. For the hand, to take an example, requires one 

 property to enable it to effect pressure, and another and different 

 646 b. 



