EXTENSION OF LAWS TO ADJACENT CASES. 331 



beyond the local limits within which it has been fonnd true by obser- 

 vation, the cases to which it is thus extended must be such as are pre- 

 sumably within the influence of the same individual agents. If" we 

 discovered a now planet ^vithin the known hounds of "the solar system 

 (or even beyond those bounds, but indicating its comiexion with the 

 system by revolving round the sun), we might conclude, with great prob- 

 ability, that it revolves upon its axis. For all the known planets do so ; 

 and thi-s uniformity points to some common cause, antecedent to the 

 first records of astronomical observation: and although the nature of 

 this cause can only be matter of conjecture, yet if it be, as is not 

 unlikely (and as Laplace's theory suggests,) one and the same indi- 

 vidual impulse given to all the bodies at once, that cause, acting at 

 the extreme points of the space occupied by the sun and planets, 

 must, unless defeated by some counteracting cause, have acted at 

 every intermediate point, and probably somewhat beyond : and there- 

 fore acted, in all probability, upon the supposed newly-discovered 

 planet. 



When, therefore, effects which are always found conjoined, can be 

 traced with any probability to an identical (and not merely a similar) 

 origin, we may with great 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 possibility of coun- 

 teracting causes in some portion of the field. Still more confidently 

 may we do so when the law is not. merely empirical ; when the ph-e- 

 nomena which we find conjoined are effects of ascertained causes, 

 from the laws of which the conjunction of their effects is deducible. 

 In that case, we may both extend the derivative uniformity over a 

 larger space, and with less deduction for the chance of counteracting 

 causes. The first, because instead of the local boundaries of our ob- 

 servation of the fact itself^ we may include the extreme boundaries of 

 the ascertained influence of its causes. Thus the succession of day and 

 night, we know, holds true of all the bodies of the solar system except 

 the sun himself; but we know this only because we are acquainted 

 with the causes: if we were not, we could not extend the proposition 

 beyond the orbits of the earth and moon, at bot"h extremities of which 

 we have the evidence of observation for its truth. With respect to the 

 probability of counteracting causes, it has been seen that this calls for 

 a greater abatement of confidence, in proportion to our ignorance of 

 the causes on which the phenomena depend. On both accounts, there- 

 fore, a derivative law which we know how to resolve, is susceptible of 

 a greater extension to cases adjacent in place, than a merely em- 

 pirical law. 



