54. On examining Locke’s second rehearsal of his theory in the fourth book of the Essay--that ‘On Knowledge’--we are led to this result quite as inevitably as in the book ‘On Ideas.’ He has a special chapter on the ‘reality of human knowledge,’ where he puts the problem thus:--‘It is evident the mind knows not things immediately, but only by the intervention of the ideas it has of them. Our knowledge therefore is real only so far as there is a conformity between our ideas and the reality of things. But what shall be here the criterion? How shall the mind, when it perceives nothing but its own ideas, know that they agree with things themselves?’ (Book IV. chap. iv. sec. 3.) It knows this, he proceeds to show, in the case of simple ideas, because ‘since the mind can by no means make them to itself, they must be the product of things operating on the mind in a natural way. ... Simple ideas are not fictions of our fancies, but the natural and regular productions of things without us, really operating upon us; and so carry with them all the conformity which is intended, or which our state requires, for they represent to us things under those appearances which they are fitted to produce in us; whereby we are enabled to distinguish the sorts of particular substances,’ &c. &c. (Book IV. chap. iv. sec. 4.) The whole force of this passage depends on the notion that simple ideas are already to the subject of them not his own making, but the product of a thing, which in its relation to these ideas is a ‘particular sort of substance.’ It is the reception of such ideas, so related, that Locke calls ‘sensitive knowledge of particular existence,’ or a ‘perception of the mind, employed about the particular existence of finite beings without us.’ (Book IV. chap. ii. sec. 14.) This, however, he distinguishes from two other ‘degrees of knowledge or certainty,’ ‘intuition’ and ‘demonstration,’ of which the former is attained when the agreement or disagreement of two ideas is perceived immediately, the latter when it is perceived mediately through the intervention of certain other agreements or disagreements (less or more), each of which must in turn be perceived immediately. Demonstration, being thus really but a series of intuitions, carries the same certainty as intuition, only it is a certainty which it requires more or less pains and attention to apprehend. (Book IV. chap. ii. sec. 4.) Of the ‘other perception of the mind, employed about the particular existence of finite beings without us,’ which ‘passes under the name of knowledge,’ he explains that although ‘going beyond bare probability, it reaches not perfectly to either of the foregoing degrees of certainty.’ ‘There can be nothing more certain,’ he proceeds, ‘than that the idea we receive from an external object is in our minds; this is intuitive knowledge. But whether there be anything more than barely that idea in our minds, whether we can