106 PHYSIOLOGY. 



certainly lose the power of moving the eyes differently, which 

 we originally had. But certain young persons, before the ha- 

 bit is very strong, if they find any use in separating the motion 

 of their eyes, of directing them different ways, or of neglecting 

 the one, have the power of squinting, and in some it is ab- 

 solutely voluntary. The motions of the eyelids also corres- 

 pond ; and such is the power of habit, that some cannot shut one 

 eye and look with the other ; but there is a use in sometimes di- 

 recting the eye along a line, and for that purpose we can gene- 

 rally shut one ; but there are few people who can apply that to 

 either eye at pleasure. Unless, therefore, we preserve the prac- 

 tice of adapting our motions to their purposes, they become in- 

 dispensably connected with those sensations alone which all 

 along have given occasion to them ; and we can see why the 

 action of respiration is with as little consciousness as that of the 

 heart, and as little in the power of the will. 



" I say, * in most of what are called voluntary motions, we 

 are conscious of willing the end proposed more than the mo- 

 tions excited ; and, of the motions produced, we are conscious 

 chiefly of those of a whole member, or of the general effect, 

 and very little of the many particular motions that concur to 

 produce it.' To explain all this I must observe that human 

 actions, and several motions of the body that are performed, 

 may be considered either as rational or as instinctive. They are 

 said to be rational, when, in consequence of a certain train of rea- 

 soning, one end or purpose is chosen in preference to another; and 

 they are often also rational in choosing the means most fit to ob- 

 tain the end, in preference to others. They are instinctive, on the 

 other hand, in two respects; first, when they arise without any rea- 

 soning, in consequence of sensations producing a desire, without 

 any other end than the gratification of that desire. Thus, in the 

 case of our several appetites ; it is not to nourish the body that a 

 man eats, it is not from any reflection or reasoning that can arise, 

 nor is it from the propriety of satisfying it, in order to nourish 

 the body, but hunger consists in a blind impulse to gratify a de- 

 sire. So the venereal appetite may be sometimes directed or 

 guided ; but it does not arise from any reasoning with respect 

 to the propriety of propagating our species ; and all such mo- 

 tions are properly instinctive with us, as they are in other 



