114 THE HISTORY OF BIOLOGY 



Starting-point. On tiiis foundation he then builds up his entire conception 

 of existence on the principle that the composite should be explained from 

 its simple components. First he constructs out of the thought of man an 

 idea of God, since man's finite and imperfect personality presupposes an in- 

 finite and perfect origin; once we believe this, we must also be right in rely- 

 ing upon our mental perceptions, for God cannot have given them to us 

 without cause. Through the senses we are convinced that matter exists. The 

 simplest and therefore the most essential qualities of matter are extension, 

 divisibility, and mobility. On the other hand, form, which Aristotle, it will 

 be remembered, made his main principle, is of momentary and therefore of 

 secondary significance. Descartes also rejects the atomic theory, for it is in- 

 consistent with the principle of divisibility; nor does space exist, for every- 

 thing that exists must have extension. On these principles — extension, 

 divisibility, and mobility — Descartes bases the whole of his theory of mat- 

 ter, both inanimate and animate, and he entirely rejects the theory of final 

 causes, for it would be presumptuous to ascribe any limited purposes to 

 unfathomable and infinite divinity. The only rational explanation of the 

 universe is to regard the whole as a machine. Through vortical movements 

 within the parts of matter the latter have accumulated and become heavenly 

 bodies; and movement is all that takes place in nature. 



Life-phenomena -purely mechanical 

 On this same principle he seeks to explain the phenomena of life — that is, 

 the corporeal. These, in his view, occur purely mechanically, without the 

 intervention of any of the spiritual forces which the Aristoteleans assumed, 

 whether animal or vegetative. Confirmation of this idea of the living body 

 as a mechanism Descartes found in Harvey's discovery of the circulation of 

 the blood, which he enthusiastically upheld and to the acceptance of which 

 he powerfully contributed. This fact in itself would be sufficient to ensure 

 him a place in the history of biology. And in drawing conclusions from 

 Harvey's observations which the latter, faithful Aristotelean as he was, 

 could himself never have perceived, he formed a theory of the human body 

 as a mechanism which may be regarded as the foundation of modern physi- 

 ology. In particular, he sought to explain mechanically the function of the 

 nervous system. He believed that from the brain the so-called animal spirits 

 are conveyed through the nerves to the muscles, which are thereby set in 

 motion through the impulse given them from the brain. It is not at all neces- 

 sary that these impulses should be conscious; they may take place in the com- 

 plete absence of thought " just as in a machine." Thus mental impressions 

 can immediately call forth movements through the nerve currents' being 

 "thrown back" — that is, reflected. He has thus recognized, described, and 

 from his own point of view explained the phenomenon of reflex motion. 

 In regard to animals he believes that all their manifestations of life are the 



