EXTENSION OF LAWS TO ADJACENT CASES. 



3^5 



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 existence. 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 co-existence 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 be- 

 comes 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 adja- 

 cent) 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 pur- 

 poses of life it is to such cases alone 

 that we can almost ever have occasion 

 to extend them. 



In respect of place, it might seem 

 that a merely empirical law could not 

 be extended even to adjacent cases ; 

 that we could have no assurance of 

 its being true in any place where it 

 has not been specially observed. The 

 past duration of a cause is a guarantee 

 for its future existence, unless some- 

 thing occurs to destroy it ; but the 

 existence of a cause in one or any 



number of places, is no guarantee for 

 its existence in any other place, since 

 there is no uniformity in the collo- 

 cations of primeval causes. When, 

 therefore, an empirical law is ex- 

 tended beyond the local limits within 

 which it has been found true^by ob- 

 servation, the cases to which it is thus 

 extended must be such as are presum- 

 ably within the influence of the same 

 individual agents. If we discover a 

 new planet within the known bounds 

 of the solar system, (or even beyond 

 those bounds, but indicating its con- 

 nection with the system by revolving 

 round the sun,) we may conclude, 

 with great probability, that it revolves 

 on its axis. For all the known planets 

 do so ; and this uniformity points to 

 some common cause antecedent to 

 the first records of astronomical ob- 

 servation : and though the nature of 

 this cause can only be matter of con- 

 jecture, yet if it be, as is not unlikely, 

 and as Laplace's theory supposes, not 

 merely the same kind of cause, but 

 the same individual cause, (such as an 

 impulse given to all the bodies at 

 once,) that cause, acting at the ex- 

 treme points of the space occupied by 

 the sun and planets, is likely, unless 

 defeated by some counteracting cause, 

 to have acted at every intermediate 

 point, and probably somewhat beyond; 

 and therefore acted, in all probability, 

 upon the supposed newly-discovered 

 planet. 



When, therefore, effects which arti 

 always found conjoined can be traced 

 with any probability to an identical 

 (and not merely a similar) origin, we 

 may with the same probability extend 

 the empirical law of their conjunction 

 to all places within the extreme local 

 boundaries within which the fact has 

 been observed ; subject to the possi- 

 bility of counteracting causes in some 

 portion of the field. Still more con- 

 fidently may we do so when the law 

 is not merely empirical ; when the 

 phenomena which we find conjoined 

 are effects of ascertained causes, from 

 the laws of which the conjunction of 

 their effects is deducible. In that 



