VI POSSIBILITIES AND IMPOSSIBILITIES 197 



demonstration differs from mine. But, if it be 

 impossible to demonstrate the non-existence of a 

 " substance " of mental phenomena that is, of a 

 soul independent of material "substance"; if the 

 idea of such a " soul " is " intelligible and can be 

 distinctly conceived/' then it follows that it is not 

 justifiable to talk of demons as " impossibilities." 

 The idea of their existence implies no more " con- 

 tradiction " than does the idea of the existence of 

 pathogenic microbes in the air. Indeed, the 

 microbes constitute a tolerably exact physical 

 analogue of the " powers of the air " of ancient 

 belief. 



Strictly speaking,! am unaware of any thing that 

 has a right to the title of an " impossibility " 

 except a contradiction in terms. There are 

 impossibilities logical, but none natural. A " round 

 square/' a " present past/' " two parallel lines that 

 intersect/' are impossibilities, because the ideas 

 denoted by the predicates, round, present, intersect, 

 are contradictory of the ideas denoted by the 

 subjects, square, past, parallel. But walking on 

 water, or turning water into wine, or "procreation 

 without male intervention, or raising the dead, are 

 plainly not " impossibilities " in this sense. 



In the affirmation, that a man walked upon 

 water, the idea of the subject is not contradictory 

 of that in the predicate. Naturalists are familiar 

 with insects which walk on water, and imagination 

 has no more difficulty in putting a man in place of 



