NATURAL SCIENCE TO GENEEAL SCIENCE. 15 



with tolerable certainty that if he has to act under certain 

 conditions, he will follow the dictates of his ambition, 

 and decide on a certain line of action. But, in the first 

 place, we cannot define with absolute precision what con- 

 stitutes an ambitious man, or by what standard the inten- 

 sity of his ambition is to be measured ; nor, again, can we 

 say precisely what degree of ambition must operate in 

 order to impress the given direction on the actions of the 

 man under those particular circumstances. Accordingly. 

 we institute comparisons between the actions of the man 

 in question, as far as we have hitherto observed them, and 

 those of other men who in similar cases have acted as he 

 has done, and we draw our inference respecting his future 

 actions without being able to express either the major or 

 the minor premiss in a clear, sharply-defined form 

 perhaps even without having convinced ourselves that our 

 anticipation rests on such an analogy as I have described. 

 In such cases our decision proceeds only from a certain 

 psychological instinct, not from conscious reasoning, 

 though in reality we have gone through an intellectual 

 process identical with that which leads us to assume that 

 a newly-discovered mammal has lungs. 



This latter kind of induction, which can never be per- 

 fectly assimilated to forms of logical reasoning, nor 

 pressed so far as to establish universal laws, plays a most 

 important part in human life. The whole of the process 

 by which we translate our sensations into perceptions 

 depends upon it, as appears especially from the investiga- 

 tion of what are called illusions. For instance, when the 

 retina of the eye is irritated by a blow, we imagine we 

 see a light in our field of vision, because we have, 

 throughout our lives, felt irritation in the optic nerves 

 only when there was light in the field of vision, and have 

 become accustomed to identify the sensations of those 

 nerves with the presence of light in the field of vision^ 



