342 NECESSARY TRUTH 



576. Our perceptive powers are mainly ' innate,' that is, the 

 infant is born with senses that do not greatly develop afterwards 

 under the stimulus of use. Since, then, these powers are ' inherited/ 

 we can perceive only within that very limited range within which 

 our ancestors perceived. But our thinking powers are mainly 

 ' acquirements ' ; they develop under the stimulus of use (under 

 increase of knowledge and practice in using it) to meet all sorts of 

 contingencies ; and, therefore, by perceiving different objects (not 

 different kinds of properties) and by combining the materials 

 supplied by our perceptions differently (by perceiving or imagining 

 different relations) we can think not only in a different, but often in 

 a more far-reaching way than our ancestors. Our sciences have 

 grown in this way. It is not his senses^ his perceptive powers^ 

 but his power S) dependent on memory, of growing mentally under the 

 stimulus of use, that confers on man his intellectual pre-eminence 

 over brutes. But of this more anon. 1 



577. I do not know how it happens that real objects have 

 (for instance) extension in space and persistence in time. My 

 senses supply no data, and, therefore, in this case also, I seem to 

 have reached the limits of the thinkable. Here I cannot trace 

 cause and effect; I can only note that all things known, to me have 

 these properties. But the fact that all the real objects I have met 

 are possessed, in some degree, of extension and persistence, has 

 given me the notion that they are present in all real things. 

 That notion, however, may not be correct, for real objects lacking 

 them may exist and I may be unaware of them because my senses 

 are incapable of being influenced by their kinds of qualities. It is 

 true that we can use a form of words and declare that a point is 

 that which has no magnitude no extension ; but such a point, a 

 thing without qualities that we can perceive, cannot really be thought 

 of. 2 We do not conceive a new thing when we use that form of 

 words ; we merely try to strip a thing we know of the qualities by 

 which we know it. I seem, then, to have reached the notion that 

 all real bodies have extension and persistence by an induction 

 from a ' simple enumeration ' ; that is, I have concluded that what 

 has always held good within my experience always would hold 

 good were my experience universal. In the same way I have 

 reached the notion that all bodies in nature are susceptible of 

 movement (of having their spatial relations to one another altered), 

 and the notion that matter is impenetrable (that no two bodies in 

 nature can occupy the same part of space at the same instant of 



1 See 624, et seq. * See J. S. Mill, Logic t Bk. II. chap, v., I. 



