INDUCTION. 169 



moment, that our most careful inferences can be fulfilled. 

 All predictions, all inferences which reach beyond their 

 data, are purely hypothetical, and proceed on the assump- 

 tion that new events will conform to the conditions 

 detected in our observation of past events. No experience 

 of finite duration can be expected to give an exhaustive 

 knowledge of all the forces which are in operation. There 

 is thus a double uncertainty ; even supposing the Uni- 

 verse as a whole to proceed unchanged, we do not really 

 know the Universe as a whole. Comparatively speaking 

 we know only a point in its infinite extent, and a moment 

 in its infinite duration. We cannot be sure, then, that our 

 observations have not escaped some fact, which will cause 

 the future to be apparently different from the past ; nor 

 can we be sure that the future really will be the outcome 

 of the past. We proceed then in all our inferences to 

 unexamined objects and times on the assumptions 



1 . That our past observation gives us a complete know- 



ledge of what exists. 



2. That the conditions of things which did exist will 



continue to be the conditions of things which will 



exist. 



We shall often need to illustrate the character of our 

 knowledge of nature by the simile of a ballot-box, so 

 often employed by mathematical writers in the theory of 

 probability. Nature is to us like an infinite ballot-box, 

 the contents of which are being continually drawn, ball 

 after ball, and exhibited to us. Science is but the careful 

 observation of the succession in which balls of various 

 character usually present themselves ; we register the 

 combinations, notice those which seem to be excluded from 

 occurrence, and from the proportional frequency of those 

 which usually appear we infer the probable character of 

 future drawings. But under such circumstances certainty 

 of prediction depends on two conditions : 



