132 HUME v 



But this loose employment of the term " in- 

 stinct " really accords with the nature of the 

 thing; for it is wholly impossible to draw any 

 line of demarcation between reflex actions and 

 instincts. If a frog, on the flank of which a little 

 drop of acid has been placed, rubs it off with the 

 foot of the same side; and, if that foot be held, 

 performs the same operation, at the cost of much 

 effort, with the other foot, it certainly displays a 

 curious instinct. But it is no less true that the 

 whole operation is a reflex operation of the spinal 

 cord, which can be performed quite as well when 

 the brain is destroyed; and between which and 

 simple reflex actions there is a complete series of 

 gradations. In like manner, when an infant 

 takes the breast, it is impossible to say whether 

 the action should be rather termed instinctive or 

 reflex. 



What are usually called the instincts of animals 

 are, however, acts of such a nature that, if they 

 were performed by men, they would involve the 

 generation of a series of ideas and of inferences 

 from them; and it is a curious, apparently an 

 insoluble, problem whether they are, or are not, 

 accompanied by cerebral changes of the same 

 nature as those which give rise to ideas and 

 inferences in ourselves. When a chicken picks up 

 a grain, for example, are there, firstly, certain 

 sensations, accompanied by the feeling of relation 

 between the grain and its own body; secondly, a 



