EXTENSION OF LAWS TO ADJACENT CASES. 83 



for five thousand years, we could infer that the unknown 

 causes on which the conjunction is dependent had existed 

 undiminished and uncounteracted during the same period. The 

 same conclusions, therefore, would follow as in the preceding 

 case ; except that we should only know that during five thou- 

 sand years nothing had occurred to defeat perceptibly this 

 particular effect ; while, when we know the causes, we have the 

 additional assurance, that during that interval no such change 

 has been noticeable in the causes themselves, as by any degree 

 of multiplication or length of continuance could defeat the 

 effect. 



To this must be added, that when we know the causes, we 

 may be able to judge whether there exists any known cause 

 capable of counteracting them ; while as long as they are un- 

 known, we cannot be sure but that if we did know them, we 

 could predict their destruction from causes actually in exis- 

 tence. A bedridden savage, who had never seen the cataract 

 of Niagara, but who lived within hearing of it, might imagine 

 that the sound he heard would endure for ever; but if he 

 knew it to be the effect of a rush of waters over a barrier of 

 rock which is progressively wearing away, he would know 

 that within a number of ages which may be calculated, it will 

 be heard no more. In proportion, therefore, to our ignorance 

 of the causes on which the empirical law depends, we can be 

 less assured that it will continue to hold good ; and the 

 farther we look into futurity, the less improbable is it that 

 some one of the causes, whose coexistence gives rise to the 

 derivative uniformity, may be destroyed or counteracted. 

 With every prolongation of time, the chances multiply of such 

 an event, that is to say, its non-occurrence hitherto becomes 

 a less guarantee of its not occurring within the given time. 

 If, then, it is only to cases which in point of time are adjacent 

 (or nearly adjacent) to those which we have actually observed, 

 that any derivative law, not of causation, can be extended 

 with an assurance equivalent to certainty, much more is this 

 true of a merely empirical law. Happily, for the purposes of 

 life it is to such cases alone that we can almost ever have 

 occasion to extend them. 



62 



