186 DESCARTES' DISCOURSE ON METHOD iv 



which constitute the physical basis of light, sound, 

 and heat, are transmuted into affections of nervous 

 matter by the sensory organs. These affections 

 are, so to speak, a kind of physical ideas, which 

 are retained in the central organs, constituting 

 what might be called physical memory, and may 

 be combined in a manner which answers to asso- 

 ciation and imagination, or may give rise to 

 muscular contractions, in those " reflex actions " 

 which are the mechanical representatives of 

 volition. 



Consider what happens when a blow is aimed at 

 the eye. 1 Instantly, and without our knowledge 

 or will, and even against the will, the eyelids close. 

 What is it that happens ? A picture of the rapidly- 

 advancing fist is made upon the retina at the back 

 of the eye. The retina changes this picture into 

 an affection of a number of the fibres of the optic 

 nerve ; the fibres of the optic nerve affect certain 

 parts of the brain ; the brain, in consequence, 

 affects those particular fibres of the seventh nerve 

 which go to the orbicular muscle of the eyelids ; 

 the change in these nerve-fibres causes the mus- 

 cular fibres to alter their dimensions, so as to 

 become shorter and broader ; and the result is the 

 closing of the slit between the two lids, round 

 which these fibres are disposed. Here is a pure 

 mechanism, giving rise to a purposive action, and 

 strictly comparable to that by which Descartes 

 1 Compare Traite dcs Passions, Art. xlii. and xvL. 



