SCIENCE IN THE ANCIENT WORLD 49 



with what we now hold to be the truth than the 

 conclusions of the older nature-philosophers, though, 

 in points of detail, Aristotle far surpassed them in 

 knowledge. 



With the more metaphysical aspects of Greek thought 

 we are not concerned. Yet, owing to its importance 

 in mediaeval controversy, it may be well to touch 

 lightly on Plato's doctrine of ideas, and Aristotle's 

 variation of it. 



In nature we find numberless groups of objects 

 more or less similar ; animal and vegetable species 

 may stand as examples. To explain the similarity, 

 the mind conceives a primary type to which, in 

 some way, the individuals have to conform. Now, 

 when the mind begins consciously to frame definitions 

 and to reason about them in general terms applicable 

 to any particular case, Plato found that the defini- 

 tions and reasoning were concerned with these 

 hypothetical types. All natural objects are in a con- 

 stant state of change; alone the types imagined by 

 the mind remain constant and unchangeable. Hence 

 Plato was led to the theory that these ideas of 

 the mind are the only realities. For Plato, says 

 Aristotle, " the objects of sense were additions to the 

 ideas, and named after them, for it was by partici- 

 pation in the ideas that their material namesakes 

 existed." 



To Aristotle, often immersed in the detailed study 

 of definite individual animals or other concrete objects, 

 this thorough-going idealism was not a convenient 

 attitude of mind. The influence of his master re- 

 mained, but he broke away from his extreme position. 

 While admitting the reality of the individuals, the 



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