CLASSICAL ANTIQUITY, MIDDLE AGES 33 



of the system of ideas, which has played an important part in biology; out 

 of the idea horse in contradistinction to the individual has arisen the no- 

 tion of species, and gradually likewise all higher systematical categories. It 

 is easy to realize the immense influence this has had on biological research. A 

 mass of detailed ideas in systematization — for instance, the dichotomic classi- 

 fication tables of genera and species — come from the Platonic school. And, 

 mathematician as he was, Plato further endeavoured to make his inferences as 

 conclusive as possible; the listener and the reader should realize that the 

 result of the investigation must be so and could not be otherwise. Thus for the 

 first time in the history of science he not only made assertions, but also sub- 

 mitted proofs of them, based, it is true, upon abstract reasoning, as was the 

 entire world of thought which he built up, but at least formally convincing. 



Theory of ideas 

 But if Plato, in laying the foundation of biological systematization, made 

 a powerful contribution to the progress of biology, nevertheless his activities 

 proved in other respects unfortunate for it. The enunciation of the world of 

 ideas as the true essence of being led to the underestimation of nature and of 

 the senses by which man observes nature. Plato realized the relativity and 

 limitation of observation through the senses,^ but not the arbitrariness to 

 which abstract thinking may lead if it is not controlled by observations. In 

 the Timaus he expressly states that no true knowledge is to be acquired 

 through the observations of the senses, but only a pleasure to the eye suitable 

 for a diversion — a statement which has been repeated after him by innumer- 

 able idealistic philosophers, both major and minor. Plato, the creator and 

 perhaps also, so far, the greatest upholder of idealistic philosophy, is like- 

 wise responsible for the contempt with which this trend of thought, which 

 otherwise deserves such high praise for contributing towards the develop- 

 ment of human thought, regarded natural philosophy. History indeed shows 

 that the more the idealistic philosophy governed man's desire for knowledge, 

 the greater became his indifference to the study of nature. In a far greater 

 degree than the traditional religions, idealistic philosophy has been the an- 

 tipodes of natural science. 



On the whole, Plato's disciples followed in their master's footsteps. 

 Of the exact sciences mathematics interested them most and they worked at it 

 with great energy; otherwise, like many other schools of thought emanating 

 from the Socratic circle, they mostly occupied themselves with ethical ques- 

 tions. Even in Plato's lifetime, however, one of his disciples in the Academy 

 had begun to discard his ideas in a number of essentials and started to guide 

 philosophy in a new direction. This man was Aristotle, the greatest biologist 

 of antiquity and one of the most many-sided natural philosophers of all time. 



^ As a matter of fact, Democritus had already realized this, but was unable to develop the 

 thought further. 



