410 INDUCTION. 



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 so, 

 provided the sun rises above the horizon. If the sun 

 ceased to rise, which, for aught we know, may be per- 

 fectly 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 

 combination of antecedents could be indefinitely pro- 

 longed, 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 exist- 

 ence 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 superfluous circum- 

 stance, constitutes the cause. This is what writers 

 mean when they say that the notion of cause in- 

 volves the idea of necessity. If there be any mean- 

 ing which confessedly belongs to the term necessity, it 

 is unconditionalness. That which is necessary, that 

 which must be, means that which will be, whatever 



* I use the words " straight line" for brevity and simplicity. 

 In reality the line in question is not exactly straight, for, from the 

 effects 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 realizing, though but to a 

 limited extent, the coveted desideratum of seeing round a corner. 



