xiv.J ON DESCARTES' "DISCOURSE* 335 



it is certain that the modes of motion which constitute 

 the physical basis of light, sound, and heat, are trans- 

 muted 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 associa- 

 tion and imagination, or may give rise to muscular 

 contractions, in those "reflex actions' which are the 

 mechanical representatives of volitions. 



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 muscular fibres to change 

 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 supposes 

 his waterwork Diana to be moved. But we may go 

 further, and inquire whether our volition, in what we term 

 voluntary action, ever plays any other part than that of 

 Descartes' engineer, sitting in his office, and turning this 

 tap or the other, as he wishes to set one or another 

 machine in motion, but exercising no direct influence 

 upon the movements of the whole. 



Our voluntary acts consist of two parts : firstly, we 



1 Compare "Traite des Passions," Art. XIII. and XVX 



