Present sensation gives knowledge of existence. 57. With that curious tendency to reduplication, which is one of his characteristics, Locke, after devoting a chapter to the ‘reality of human knowledge,’ of which the salient passage as to simple ideas has been already quoted, has another upon our ‘knowledge of existence.’ Here again it is the sensitive knowledge of things actually present to our senses, which with him is merely a synonym for the simple idea, that is the prime criterion. (Book IV. chap. iii. secs. 5 and 2, and chap. ii. sec. 2.) After speaking of the knowledge of our own being and of the existence of a God (about which more will be said below), he proceeds, ‘No particular man can know the existence of any other being, but only when, by actually operating upon him, it makes itself perceived by him. For the having the idea of anything in our mind no more proves the existence of that thing, than the picture of a man evidences his being in the world, or the visions of a dream make thereby a true history. It is therefore the actual receiving of ideas from without, that gives us notice of the existence of other things, and makes us know that something doth exist at that time without us, which causes that idea in us, though perhaps we neither know nor consider how it does it; for it takes not from the certainty of our senses and the ideas we receive by them, that we know not the manner wherein they are produced; _e.g._ whilst I write this, I have, by the paper affecting my eyes, that idea produced in my mind, which, whatever object causes, I call _white_; by which I know that the quality or accident (_i.e._ whose appearance before my eyes always causes that idea) doth really exist, and hath a being without me. And of this the greatest assurance. I can possibly have, and to which my faculties can attain, is the testimony of my eyes, which are the proper and sole judges of this thing, whose testimony I have reason to rely on, as so certain, that I can no more doubt whilst I write this, that I see white and black, and that something really exists that causes that sensation in me, than that I write and move my hand.’ (Book IV. chap. xi. secs. 1, 2.) Reasons why its testimony must be trusted.