HYPOTHESES. 5 



but introduce, as one element of the explanation, the proposi- 

 tion that some antecedent or other produces the sensation of 

 white. 



The ideal limit, therefore, of the explanation of natural 

 phenomena (towards which as towards other ideal limits we 

 are constantly tending, without the prospect of ever completely 

 attaining it) would he to show that each distinguishable 

 variety of our sensations, or other states of consciousness, has 

 only one sort of cause ; that, for example, whenever we per- 

 ceive a white colour, there is some one condition or set of con- 

 ditions which is always present, and the presence of which 

 always produces in us that sensation. As long as there are 

 several known modes of production of a phenomenon, (several 

 different substances, for instance, which have the property of 

 whiteness, and between which we cannot trace any other 

 resemblance,) so long it is not impossible that one of these 

 modes of production may be resolved into another, or that all 

 of them may be resolved into some more general mode of pro- 

 duction not hitherto recognised. But when the modes of 

 production are reduced to one, we cannot, in point of simplifi- 

 cation, go any further. This one may not, after all, be the 

 ultimate mode; there may be other links to be discovered 

 between the supposed cause and the effect ; but we can only 

 further resolve the known law, by introducing some other law 

 hitherto unknown ; which will not diminish the number of 

 ultimate laws. 



In what cases, accordingly, has science been most successful 

 in explaining phenomena, by resolving their complex laws 

 into laws of greater simplicity and generality ? Hitherto chiefly 

 in cases of the propagation of various phenomena through 

 space : and, first and principally, the most extensive and 

 important of all facts of that description, the fact of motion. 

 Now this is exactly what might be expected from the principles 

 here laid down. Not only is motion one of the most universal 

 of all phenomena, it is also (as might be expected from that cir- 

 cumstance) one of those which, apparently at least, are produced 

 in the greatest number of ways ; but the phenomenon itself is 

 always, to our sensations, the same in every respect but degree. 



