So much must surely be conceded to me, for that white, being white, cannot be also black is not nearly so certain as that idea and sensation, being two distinct things, idea of idea cannot be idea of sensation. The concession, indeed, is likely enough to be accompanied by an exclamation of wonder that so microscopic a flaw in an elaborate exposition should be thought worth pointing out; but Hume himself would certainly not have so retorted. Of the doctrine which I am impugning, viz., that every _idea_ is copied from some preceding _sensation_, he had spoken as follows:--'Those who would assert that this position is not universally true, nor without exception, have only one, and that an easy, method of refuting it, by producing the idea which, in their opinion, is not derived from this source. It will then be incumbent on us, if we would maintain our doctrine, to produce the impression or lively perception which corresponds to it.' He was much too candid not to have acknowledged that this challenge of his had been fairly and fully met. He was not a man to refuse to own himself refuted when, after distinctly intimating that the production of one single idea, having no perception correspondent to it amongst those which we experience 'when we see, or hear, or feel, or love, or hate, or will, or desire,' would suffice for his entire refutation, he found such an idea produced. He knew too well also to what enormous errors of thought minute errors of expression may lead, to disregard any speck of inaccuracy in any one of his definitions. The apparently slight oversight committed by him on this occasion will, indeed, be presently seen to have sensibly contributed to lead him subsequently into a mistake of no small practical moment.