PHILOSOPHICAL AND PSYCHOLOGICAL ASPECTS 227 



compounded within our minds upon the basis of sense- 

 impressions are the only trustworthy facts, aside from the 

 subjective facts of the individual's existence and his knowl- 

 edge of the modes by which the mind operates. One comes, 

 therefore, to the belief that there is only one kind of knowl- 

 edge concerning external realities, and only one way by 

 which it is acquired. 



Scientific reality, accordingly, consists in the fact, first, 

 that we get the expected result in consciousness when we 

 again experience a given group of sense-impressions; and 

 second, that the mental states of other individuals appear to 

 follow a similar course. One check upon the reality of any 

 sense-impression and upon the validity of our conclusions 

 therefrom is the commonness existing in our minds over a 

 period of time and the commonness which appears to exist 

 between the impressions in our minds and those within the 

 minds of other individuals. A check may also be obtained 

 by using other senses than the one temporarily in operation, 

 as when we verify sight by touch. Herein lies the difference 

 between reality, on the one hand, and delusion, illusion, and 

 hallucination on the other. The vividness of those latter ex- 

 periences, which are peculiar to the individual is not denied, 

 what is insisted upon is the difference between that which is 

 the product of a single mind and cannot be produced by 

 other minds in a normal state, and that product which can 

 be shared by many minds on the basis of a common under- 

 standing of sense-impressions. 



In this connection the question arises whether the human 

 mind has methods of obtaining knowledge regarding external 

 realities other than the one above described, whether what 

 is vaguely termed intuition, insight, revelation, or the like 

 gives anything that can be dignified by the term knowledge. 

 According to the scientific point of view, these mystical 

 short-cuts to knowledge are valueless, because they are so 

 dissimilar in different individuals that they fail to give suf- 

 ficient commonness when comparisons are instituted. 



