222 



INDUCTION. 



between that body and the part of 

 the earth where we are situated, are 

 the sole conditions ; and the union 

 of these, without the addition of any 

 superfluous circumstance, constitutes 

 the causa This is what writers mean 

 when they say that the notion of 

 cause involves the idea of necessity. 

 If there be any meaning which con- 

 fessedly belongs to the term necessity, 

 it is unconditionatness. That which 

 is necessary, that which must be, 

 means that which will be, whatever 

 supposition we may make in regard 

 to all other things. The succession 

 of day and night evidently is not 

 necessary in this sense. It is con- 

 ditional on the occurrence of other 

 antecedents. That which will be fol- 

 lowed by a given consequent when, 

 and only when, some third circum- 

 stance also exists, is not the cause, 

 even though no case should ever have 

 occurred in which the phenomenon 

 took place without it. 



Invariable sequence, therefore, is 

 not synonymous with causation, un- 

 less the sequence, besides being in- 

 variable, is unconditional. There are 

 sequences, as uniform in past experi- 

 ence as any others whatever, which 

 yet we do not regard as cases of 

 causation, but as conjunctions in some 

 sort accidental. Such, to an accurate 

 thinker, is that of day and night. 

 The one might have existed for any 

 length of time, and the other not have 

 followed the sooner for its existence ; 

 it follows only if certain other ante- 

 cedents exist ; and where those ante- 

 cedents existed, it would follow in 

 any case. No one, probably, ever 

 called night the cause of day ; man- 

 kind must BO soon have arrived at 

 the very obvious generalisation, that 

 the state of general illumination which 

 we call day would follow from the pre- 



brevity and simplicity. In reality the line 

 in question is not exactly straight, for, 

 from the effect of refraction, we actually 

 see the sun for a short interval during 

 which the opaque mass of the earth is 

 interposed in a direct line between the sun 

 and our eyes ; thus realising, though but 

 to a limited extent, the coveted desidera- 

 tum of seeing round a corner. 



eence of a sufficiently luminous body, 

 whether darkness had preceded or not. 



We may define, therefore, the cause 

 of a phenomenon to be the ante- 

 cedent, or the concurrence of antece- 

 dents, on which it is invariably and 

 unconditionally consequent. Or if we 

 adopt the convenient modification of 

 the meaning of the word cause which 

 confines it to the assemblage of positive 

 conditions without the negative, then 

 instead of "unconditionally," we must 

 say, "subject to no other than nega- 

 tive conditions." 



To some it may appear, that the 

 sequence between night and day being 

 invariable in our experience, we have 

 as much ground in this case as ex- 

 perience can give in any case for 

 recognising the two phenomena as 

 cause and effect ; and that to say 

 that more is necessary — to require a 

 belief that the succession is uncon- 

 ditional, or, in other words, that it 

 would be invariable under all changes 

 of circumstances — is to acknowledge 

 in causation an element of belief not 

 derived from experience. The answer 

 to this is, that it is experience itself 

 which teaches us that one uniformity 

 of sequence is conditional and another 

 unconditional. When we judge that 

 the succession of night and day is a 

 derivative sequence, depending on 

 something else, we proceed on grounds 

 of experience. It is the evidence of 

 experience which convinces us that 

 day could equally exist without being 

 followed by night, and that night 

 could equally exist without being fol- 

 lowed by day. To say that these 

 beliefs are "not generated by our 

 mere observation of sequence," * is 

 to forget that twice in every twenty- 

 four hours, when the sky is clear, we 

 have an experimentum crucis that the 

 cause of day is the sun. We have an 

 experimental knowledge of the sun 

 which justifies us on experimental 

 grounds in concluding, that if the 

 sun were always above the horizon 

 there would be day, though there had 



* Second Burnet Prize Essay, by Principal 

 TuUoch, p. 25, 



