LAW OF CAUSATION. 397 



by an agency purely physical, provided it be much more familiar 

 than that which it is invoked to explain. To Thales and 

 Anaximenes, it appeared inconceivable that the antecedents 

 which we see in nature, should produce the consequents ; but 

 perfectly natural that water, or air, should produce them. The 

 writers whom I oppose declare this inconceivable, but can con- 

 ceive that mind, or volition, is per se an efficient cause : while 

 the Cartesians could not conceive even that, but peremptorily 

 declared that no mode of production of any fact whatever was 

 conceivable, except the direct agency of an omnipotent being. 

 Thus giving additional proof of what finds new confirmation 

 in every stage of the history of science : that both what 

 persons can, and what they cannot, conceive, is very much an 

 affair of accident, and depends altogether on their experience, 

 and their habits of thought ; that by cultivating the requisite 

 associations of ideas, people may make themselves unable to 

 conceive any given thing ; and may make themselves able to 

 conceive most things, however inconceivable these may at first 

 appear : and the same facts in each person's mental history 

 which determine what is or is not conceivable to him, deter- 

 mine also which among the various sequences in nature will 

 appear to him so natural and plausible, as to need no other 

 proof of their existence ; to be evident by their own light, 

 independent equally of experience and of explanation. 



By what rule is any one to decide between one theory of 

 this description and another? The theorists do not direct us 

 to any external evidence ; they appeal each to his own sub- 

 jective feelings. One says, the succession C, B, appears to me 

 more natural, conceivable, and credible per se, than the succes- 

 sion A, B ; you are therefore mistaken in thinking that B 

 depends upon A ; I am certain, though I can give no other 

 evidence of it, that C comes in between A and B, and is the 

 real and only cause of B. The other answers the successions 

 C, B, and A, B, appear to me equally natural and conceivable, 

 or the latter more so than the former: A is quite capable of 

 producing B without any other intervention. A third agrees 

 with the first in being unable to conceive that A can produce B, 

 but finds the sequence D, B, still more natural than C, B, or 

 of nearer kin to the subject matter, and prefers his D theory 



