392 INDUCTION. 



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 equiv- 

 alent to certainty, much more is this true of a merely empirical law. Hap- 

 pily, for the purposes 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 something 

 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 collocations of primeval causes. When, therefore, an em- 

 pirical law is extended beyond the local limits within which it has been 

 found true by observation, the cases to which it is thus extended must be 

 such as are presumably 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 connection 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 uniform- 

 ity points to some common cause, antecedent to the first records of astro- 

 nomical observation ; and though the nature of this cause can only be mat- 

 ter of conjecture, 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 extreme points of the space occupied by the sun and planets, is likely, 

 unless defeated by some counteracting cause, to have acted at every inter- 

 mediate point, and probably somewhat beyond ; and therefore 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 the same probability extend the empirical law of their conjunc- 

 tion to all places within the extreme local boundaries within which the fact 

 has been observed, subject to the possibility of counteracting causes in some 

 portion of the field. Still more confidently may we do so when the law is 

 not merely empirical; when the phenomena which we find conjoined are 

 effects of ascertained causes, fi-om the laws of which the conjunction of their 

 effects is deducible. In that case, we may both extend the derivative uni- 

 formity over a larger space, and with less abatement for the chance of 

 counteracting causes. The first, because instead of the local boundaries of 

 our observation 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 itself; 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 both 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, therefore, a derivative law which 

 we know how to resolve, is susceptible of a greater extension to cases ad- 

 jacent in place, than a merely empirical law. 



