LAW OF CAUSATION. 203 



synonymous with " tlie antecedent which it invariably has followed in 

 our past experience." Such a mode of viewing causation would be 

 liable to the objection very plausibly urged by Dr. Reid, namely, that 

 according to this doctrine night must be the cause of day, and day the 

 cause of night ; since these phenomena have invariably succeeded one 

 another from the beginning of the world. But it is necessary to our 

 using the word cause, that we should believe not only that the ante- 

 cedent always has been followed by the consequent, but that, as long 

 as the present constitution of things endurcss, it always will be so. And 

 this would not be true of day and night. We do not believe that night 

 will be followed by day under any imaginable circumstances, but only 

 that it will be ?>o,prov.idcd the sun rises above the horizon. If the sun 

 ceased to rise, which, for aught we know, may be perfectly compatible 

 with the general laws of matter, night would be, or might be, eternal. 

 On the other hand, if the sun is above the horizon, his light not extinct, 

 and no opaque body between us and him, we believe firmly that unless 

 a change takes place in the properties of matter, this combination of 

 antecedents will be followed by the consequent, day ; that if the com- 

 bination of antecedents could be indefinitely prolonged, it would be 

 always day ; and that if the same combination had always existed, it 

 would always have been day, quite independently of night as a previous 

 condition. Therefore is it that we do not call night the cause, nor even 

 a condition of day. The existence of the sun (or some such luminous 

 body), and there being no opaque medium in a straight line* 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 super- 

 fluous circumstance, constitutes the cause. This is what writers mean 

 when they say that the notion of cause involves the idea of necessity. 

 If there be any meaning which confessedly belongs to the term neces- 

 sity, it is unconditionalmss. 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 conditional upon the occurrence of 

 other antecedents. That which wDl be followed by a given consequent 

 when, and only when, some third circumstance also exists, is not the 

 cause, oven although no case should have ever occuiTed in which the 

 phenomenon took place without it. 



Invariable sequence, therefore, is not synonymous with causation, 

 unless the sequence, besides being invariable, is unconditional. There 

 are sequences as uniform in past experience as any others whatever, 

 which yet we do not regard as cases of causation, but as conjunctions, 

 in some sort accidental. Such, to a philosopher, 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 fi)llows only if 

 certain other antecedents exist ; and where those antecedents existed, 

 it would follow in any case. No one, probably, ever called night the 

 cause of day ; mankind must so soon have arrived at the very obvious 

 generalization, that the state of general illumination which we call day 



* I use the words " straight line" for brevity and simplicity. In reality the line in ques- 

 tion is not exactly straight, for, from the effects of refraction, we actually see the snn for a 

 short interval during which the opaque mass of the earth is interposed in a direct line be- 

 tween the sun and our eyes ; thus realizing, though but to a limited extent, the coveted 

 desideratum of seeing round a corner. 



