SPACE AND TIME 165 



faculty presents sense-impressions to us as separated into 

 groups, and further, that though this separation is most 

 serviceable for practical purposes, it is not very exactly 

 and clearly defined " at the limits " (p. 66). How do we 

 represent in thought, in conception, this separation into 

 groups which results from our mode of perception ? The 

 answer is : We conceive groups of sense-impressions to be 

 bounded by surfaces, to be limited by straight or curved 

 lines. Thus our consideration of conceptual space leads 

 us at once to a discussion of surfaces and lines — to a 

 study, in fact, of Geometry. 



Several important problems at once present themselves 

 for investigation. In the first place, have these surfaces 

 and lines a real existence in the world of perception ? 

 Are they phenomena ? Or are they ideal modes whereby 

 we analyse the manner in which we perceive phenomena ? 

 In the second place, if they should be only ideals of 

 conception, what is the historical process by which they 

 have been reached ? What is their ultimate root in 

 perception ? 



Now there is at this stage an important remark to be 

 made, namely, that zvJiat is imperceptible is not therefore 

 inconceivable. This remark is all the more necessary, for 

 it seems directly opposed to the healthy scepticism of 

 Hume.^ Yet unless it be true the whole fabric of exact 

 science falls to the ground, neither the concepts of 

 geometry, nor those of mechanics, would be of service ; 

 for example, the circle and the motion of a point would 

 be absurdities if, being imperceptible, they were really 

 inconceivable. The basis of our conceptions doubtless 

 lies in perceptions, but in imagination we can carry on 

 perceptual processes to a limit which is itself not a 

 perception ; we can further associate groups of stored 

 sense - impresses, and form ideas which correspond to 

 nothing in our perceptual experience. 



Here a word of caution is, however, very necessary. 

 Because we conceive a thing, we must not argue that it 



^ See especially the Treatise on Hutnan Nature, part ii. Of the Ideas 

 of Space and Time. Green and Grose : Hiime^s Works, vol. i. pp. 334-371. 



