136 THE GRAMMAR OF SCIENCE 



and we may use these conclusions as a means of testing 

 our perceptions. So far as they are confirmed, the theory 

 remains valid as a description ; if, on the other hand, our 

 sense-impressions differ from these conclusions, the con- 

 clusions have just as much mental necessity, but the theory 

 while valid for the mind is not valid as a description of 

 the routine of perceptions. It is only the very great 

 probability deduced from past experience of routine that 

 enables us to speak of the " invariable order of the 

 universe," or enables scientists to assert that facts which 

 have hitherto proved obstinate will be ultimately embraced 

 by the already well-established laws of nature. Not in 

 the field of causation, but in that of conception do we 

 deal with certainties. 



^ 12. — Routine in Perception is a necessary condition 



of Knozvledge 



While in the nature of perceptions themselves there 

 appears nothing tending to enforce an order D, E, F, G 

 rather than F, G, D, E, there is still a real need, if thought 

 is to be possible, that the perceptive faculty should always 

 repeat the sequence in the same order. In other words, 

 repetition or routine is an essential condition of thought ; 

 the actual order of the sequence is immaterial, but what- 

 ever it may be, it must repeat itself if knowledge is to be 

 possible. We express this briefly in the law : That the same 

 (Chapter V. ^ 6) set of causes is alzvays accompanied' by the 

 same effect. That the future will be like our experience of 

 the past is the sole condition under which we can predict 

 what is about to happen and so guide our conduct. But 

 thought has been evolved in the struggle for existence as 

 a guide to conduct, and therefore could not have been 

 evolved had this condition been absent. If after the 

 sense-impressions D, E, F, G, the sense-impression H does 

 not uniformly follow, but A, J, or even Z, occurs equally 

 often, then knowledge becomes impossible for us, and we 

 must cease to think. The power of thinking — or of 

 associating groups and sequences of sense -impressions, 



