NATURAL SCIENCE TO GENERAL SCIENCE. 13 



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 constitutes an 

 ambitious man, or by what standard the intensity 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 pre- 

 miss in a clear, sharply defined form perhaps even without hav- 

 ing 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 perfectly 

 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 sen- 

 sations into perceptions depends upon it, as appears especially 

 from the investigation of what are called illusions. For in- 

 stance, 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. Moreover, such is the 

 complexity of the influences affecting the formation both of 

 character in general and of the mental condition at any given 

 moment, that this same kind of induction necessarily plays a 

 leading part in the investigation of psychological processes. In 

 fact, in ascribing to ourselves free-will, that is, full power to act 

 as we please without being subject to a stern inevitable law of 



