[2] P. 363. [Book I, part II., sec. V.] [3] Above, § 206. [4] P. 364. [Book I, part II., sec. V.] His explanation implies that we have an idea virtually the same. 280. It appears then that we have an idea of ‘distance unfilled with any coloured or solid object.’ To speak of this distance as ‘imaginary’ or fictitious can according to Hume’s principles make no difference, so long as he admits, which he is obliged to do, that we actually have an idea of it; for every idea, being derived from an impression, is as much or as little imaginary as every other. And not only have we such an idea, but Hume’s account of the ‘relations’ between it and the idea of extension implies that, _as ideas of distance_, they do not differ at all. But the idea of ‘distance unfilled with any coloured or solid object’ _is_ the idea of vacuum. It follows that the idea of extension does not differ from that of vacuum, except so far as it is other than the idea of distance. But it is from the consideration of distance that Hume himself expressly derives it; [1] and so derived, it can no more differ from distance than an idea from a corresponding impression. Thus, after all, he has to all intents and purposes to admit the idea of vacuum, but saves appearances by refusing to call it extension--the sole reason for such refusal being the supposition that every idea, and therefore the idea of extension, must be a datum of sense, which the admission of an idea of ‘invisible and intangible distance’ already contradicts. [1] Part II. § 3, sub. inst. By a like device that he is able to explain the appearance of our having such ideas as Causation and Identity.