17. It is not difficult to show that to have a simple idea, according to Locke’s account of it, means to have already the conception of substance and relation, which are yet according to him ‘complex and derived ideas,’ ‘the workmanship of the mind’ in opposition to its original material, the result of its action in opposition to what is given it as passive. The equivocation in terms under which this contradiction is generally covered is that between ‘idea’ and ‘quality.’ ‘Whatever the mind perceives in itself, or is the immediate object of perception, thought, or understanding, that I call _idea_; and the power to produce that idea I call quality of the subject wherein that power is. Thus a snowball having power to produce in us the ideas of white, cold, and round, the powers to produce these ideas in us, as they are in the snowball, I call qualities; and as they are sensations or perceptions in our understandings, I call them ideas; which ideas, if I speak of sometimes as in the things themselves, I would be understood to mean those qualities in the object which produce them in us.’ (Book II. chap. viii. sec. 8.) Locke’s way of interchanging ‘idea’ and ‘quality’ and its effects. 18. An equivocation is not the less so because it is announced. It is just because Locke allows himself at his convenience to interchange the terms ‘idea’ and ‘quality’ that his doctrine is at once so plausible and so hollow. The essential question is whether the ‘simple idea,’ as the original of knowledge, is on the one hand a mere feeling, or on the other a thing or quality of a thing. This question is the crux of empirical psychology. Adopting the one alternative, we have to face the difficulty of the genesis of knowledge, as an apprehension of the real, out of mere feeling; adopting the other, we virtually endow the nascent intelligence with the conception of substance. By playing fast and loose with ‘idea’ and ‘quality,’ Locke disguised the dilemma from himself. Here again the metaphor of Impression did him yeoman’s service. The idea, or ‘immediate object of thought,’ being confused with the affection of the sensitive organs, and this again being accounted for as the result of actual impact, it was easy to represent the idea itself as caused by the action of an outward body on the ‘mental tablet.’ Thus Locke speaks of the ‘objects of our senses obtruding their particular ideas on our minds, whether we will or no.’ (Book II. chap. i. sec. 25.) This sentence holds in solution an assumption and two fallacies. The assumption (with which we have no further concern here) is the physical theory that matter affects the sensitive organs in the way of actual impact. Of the fallacies, one is the confusion between this affection and the idea of which it is the occasion to the individual; the other is the implication that this idea, as such, in its prime simplicity, recognises itself as the result of, and refers itself as a quality to, the matter supposed to cause it.