298 REASONING. 



rience has hitherto been uniform in its favour, the real evidence 

 for the supposition is not the inconceivableness, but the uni- 

 formity of experience. Now this, which is the substantial and 

 only proof, is directly accessible. We are not obliged to presume 

 it from an incidental consequence. If all past experience is 

 in favour of a belief, let this be stated, and the belief openly 

 rested on that ground : after which the question arises, what 

 that fact may be worth as evidence of its truth ? For uni- 

 formity of experience is evidence in very different degrees : in 

 some cases it is strong evidence, in others weak, in others it 

 scarcely amounts to evidence at all. That all metals sink in 

 water, was an uniform experience, from the origin of the 

 human race to the discovery of potassium in the present cen- 

 tury by Sir Humphry Davy. That all swans are white, was 

 an uniform experience down to the discovery of Australia. In 

 the few cases in which uniformity of experience does amount 

 to the strongest possible proof, as with such propositions as 

 these, Two straight lines cannot inclose a space, Every event 

 has a cause, it is not because their negations are inconceivable, 

 which is not always the fact ; but because the experience, 

 which has been thus uniform, pervades all nature. It will be 

 shown in the following Book that none of the conclusions 

 either of induction or of deduction can be considered certain, 

 except as far as their truth is shown to be inseparably bound 

 up with truths of this class. 



I maintain then, first, that uniformity of past experience is 

 very far from being universally a criterion of truth. But 

 secondly, inconceivableness is still farther from being a test 

 even of that test. Uniformity of contrary experience is only 

 one of many causes of inconceivability. Tradition handed 

 down from a period of more limited knowledge, is one of the 

 commonest. The mere familiarity of one mode of production 

 of a phenomenon, often suffices to make every other mode 

 appear inconceivable. Whatever connects two ideas by a 

 strong association may, and continually does, render their 

 separation in thought impossible; as Mr. Spencer, in other 

 parts of his speculations, frequently recognises. It was not 

 for want of experience that the Cartesians were unable to con- 



