2 1. I. 



and in other similar affections and conditions, which may be passed 

 over for the present, as we are not yet prepared to treat of them 

 with clearness and precision. Now it is plain that if we discuss 

 each kind of animal separately, we shall frequently be obliged to 

 repeat the same statements over and over again ; for horse and dog 

 and man present, each and all, every one of the phenomena just 

 enumerated. A discussion therefore of these several animals 

 separately would necessarily involve frequent repetitions as to 

 characters, themselves identical, but recurring in animals speci- 

 fically distinct. (Very possibly also there may be other characters 

 which, though they present specific differences, yet come under 

 one and the same category. For instance, flying, walking, swim- 

 ming, creeping, are plainly specifically distinct, but yet are all 

 forms of animal progression.) We must, then, have some clear 

 understanding as to the manner in which our investigation is to 

 be conducted ; whether, I mean, we are first to deal with the 

 common or generical characters, and afterwards to take into con- 

 sideration special peculiarities ; or whether we are to start straight 

 off with the ultimate species. For as yet no definite rule has been 

 laid down in this matter. So also there is a like uncertainty as 

 to another point now to be mentioned. Ought the writer who 

 deals with the works of nature to follow the plan adopted by 

 the mathematicians in their astronomical demonstrations, and 

 after considering the phenomena presented by animals, and their 

 several parts, proceed subsequently to treat of the causes and the 

 reason why ; or ought he to follow some other method ? And 

 when these questions are answered, there yet remains another. 

 The causes concerned in the generation of the works of nature 

 are, as we see, more than one. There is the final cause and there 

 is the motor cause. Now we must decide which of these two 

 causes ^ comes first, which second. Plainly, however, that cause 

 is the first which we call the final one. For this is the Reason, 

 and the Reason forms the starting-point, alike in works of art 

 and in works of nature. For consider how the physician or how 

 the builder sets about his work. He starts by forming for himself 

 a definite picture, in the one case mental, in the other actual, of 

 his end — the physician of health, the builder of a house — and 

 this he holds forward as the reason and explanation of each sub- 

 sequent step that he takes, and of his acting in this or that way 

 as the case may be. Now in the works of nature the good end 

 630 b. 



