by which, in the absence of any such idea, our language about ‘pure space’ has to be accounted for, does not according to Hume’s own showing presuppose such an idea. [1] P. 358. [Book I, part II., sec. V.] [2] [Greek συγκεχυμένος (synkechymenos) = confused or jumbled-up. Tr.] How it is that we talk as if we had idea of vacuum according to Hume. 279. By vacuum he understands invisible and intangible extension. If an idea of vacuum, then, is possible at all, he argues, it must be possible for darkness and mere motion to convey it. That they cannot do so _alone_ is clear from the consideration that darkness is ‘no positive idea’ and that an ‘invariable motion,’ such as that of a ‘man supported in the air and softly conveyed along by some invisible power,’ gives no idea at all. Neither can they do so when ‘attended with visible and tangible objects.’ ‘When two bodies present themselves where there was formerly an entire darkness, the only change that is discoverable is in the appearance of these two objects: all the rest continues to be, as before, a perfect negation of light and of every coloured or tangible object’. [1] ‘Such dark and indistinguishable distance between two bodies can never produce the idea of extension,’ any more than blindness can. Neither can a like ‘imaginary distance between tangible and solid bodies.’ ‘Suppose two cases, viz. that of a man supported in the air, and moving his limbs to and fro without meeting anything tangible; and that of a man who, feeling something tangible, leaves it, and after a motion of which he is sensible perceives another tangible object. Wherein consists the difference between these two cases? No one will scruple to affirm that it consists merely in the perceiving those objects, and that the sensation which arises from the motion is in both cases the same; and as that sensation is not capable of conveying to us an idea of extension, when unaccompanied with some other perception, it can no more give us that idea, when mixed with the impressions of tangible objects, since that mixture produces no alteration upon it’. [2] But though a ‘distance not filled with any coloured or solid object’ cannot give us an idea of vacuum, it is the cause why we falsely imagine that we can form such an idea. There are ‘three relations’--_natural_ relations according to Hume’s phraseology [3]--between it and that distance which really ‘conveys the idea of extension.’ ‘The distant objects affect the senses in the same manner, whether separated by the one distance or the other; the former species of distance is found capable of receiving the latter; and they both equally diminish the force of every quality. These relations betwixt the two kinds of distance will afford us an easy reason why the one has so often been taken for the other, and why we imagine we have an idea of extension without the idea of any object either of the sight or feeling’. [4] [1] P. 362. [Book I, part II., sec. V.]