PARTS OF ANIMALS, I. i. 



philosophers endeavour to carry back their explana- 

 tions to Necessity ; but they omit to distinguish the 

 various meanings of Necessity. There is " absolute " 

 Necessity,^ which belongs to the eternal things ; and 

 there is " conditional " Necessity, which has to do 

 with everything that is formed by the processes of 

 Nature, as well as with the products of Art, such as 

 houses and so forth. If a house, or any other End, 

 is to be realized, it is necessary that such and such 

 material shall be available ; one thing must first be 

 formed, and set in motion, and then another thing ; 

 and so on continually in the same manner up to the 

 End, which is the Final Cause, for the sake of which 

 every one of those things is formed and for which it 

 exists. The things which are formed in Nature are 

 in like case. Howbeit, the method of reasoning 

 in Natural science and also the mode of Necessity 

 itself is not the same as in the Theoretical sciences. 

 (I have spoken of this matter in another treatise.^) 

 They differ in the following way.'' In the Theoretical 

 sciences, we begin with what already is ; but in 

 Natural science ^Wth what is goi?ig to be : thus, w^e say, 

 Because that which is going to be — health, perhaps, or 

 man — has a certain character, therefore of necessity 

 some particular thing, P, must be, or must be formed ; 

 not, Because P is now, or has been formed, therefore 

 the other thing (health, or man) of necessity is now 



may be a " theoretical " science, but Nature's science itself 

 is " productive." 



" The reasoning process in a " theoretical " science, e.g. 

 mathematics, begins, say, with A, and then deduces from it 

 the consequences B, C, D. In a " productive " science, 

 e.g. building, it begins with the house which is to be built, 

 D, and works backwards through the preliminary stages 

 which must be realized in order to produce the house, 

 C, B, A. Cf. below, 640 a 16 ff. 



c 59 



