There are no innate ideas "stamped upon the mind" from birth; and yet impressions of sense are not the only source of knowledge: "The mind furnishes the understanding with ideas" (Bk. 2:1:5). No distinction is implied here between "mind" and "understanding", so that the sentence might run, "the mind furnishes itself with ideas." As to what these ideas are, we are not left in doubt: they are "ideas of its own operations." When the mind acts, it has an idea of its action, that is, it is self-conscious, and, as such, is assumed to be an original source of our knowledge. Hume and Condilac both refused to admit reflection as an original source of ideas, and both, accordingly, found that they had to face the problem of tracing the growth of self-consciousness out of a succession of sensations. According to Locke, reflection is an original, rather than an independent, source of ideas. Without sensation mind would have nothing to operate upon, and therefore could have no ideas of its operations. It is "when he first has any sensation" that "a man begins to have any ideas" (Bk. 2:1:23). The operations of the mind are not themselves produced by sensation, but sensation is required to give the mind material for working on.
The ideas which sensation gives "enter by the senses simple and unmixed" (Bk. 2:2:1); they stand in need of the activity of mind to bind them into the complex unities required for knowledge. The complex ideas of substance, modes, and relations are all the product of the combining and abstracting activity of mind operating upon simple ideas, which have been given, without any connection, by sensation or reflection. Locke's account of knowledge thus has two sides. On the one side, all the material of knowledge is traced to the simple idea. On the other side, the processes which transform this crude material into knowledge are activities of mind which themselves cannot be reduced to ideas. Locke's metaphors of the tabula rasa, "white paper" (Bk. 2:2:1), and "dark room" misled his critics and suggested to some of his followers a theory very different from his own. The metaphors only illustrate what he had in hand at the moment. Without experience, no characters are written on the "tablets" of the mind; except through the "windows" of sensation and reflection, no light enters the understanding. No ideas are innate; and there is no source of new simple ideas other than those two. But knowledge involves relations, and relations are the work of the mind; it requires complex ideas, and complex ideas are mental formations. Simple ideas do not, of themselves, enter into relation and form complex ideas. Locke does not, like Hobbes before him and Hume and Condillac after him, look to some unexplained natural attraction of idea for idea as bringing about these formations. Indeed, his treatment of "the association of ideas" is an afterthought, and did not appear in the earlier editions of the Essay.
Starting from the simple ideas which we get from sensation, or from observing mental operations as they take place, Locke has two things to explain: the universal element, that is, the general conceptions with which knowledge is concerned or which it implies; and the reference to reality which it claims. With the former problem Locke deals at great length; and the general method of his exposition is clear enough. Complex ideas arise from simple ideas by the processes of combination and abstraction carried out by the mind. It would be unfair to expect completeness from his enterprise: but it cannot be denied that his intricate and subtle discussions left many problems unsolved. Indeed, this is one of his great merits. He raised questions in such a way as to provoke further enquiry. Principles such as the causal relation, apart from which knowledge of nature would be impossible, are quietly taken for granted, often without any enquiry into the grounds for assuming them. Further, the difficulty of accounting for universals is unduly simplified by describing certain products as simple ideas, although thought has obviously been at work upon them.
In this connection an important inconsistency becomes apparent in his account of the primary data of experience. It is, indeed, impossible even to name the mere particular -- the "this, here, and now" of sense -- without giving it a flavor of generality. But, at the outset, Locke tries to get as near it as possible. Simple ideas (of sensation) are exemplified by yellow, white, heat, cold, soft, hard, and so forth (Bk. 2:1:3). But, towards the end of the second book (Bk. 2:21:75), a very different list is given: extension, solidity, and mobility (from sensation); perceptivity and motivity (from reflection); and existence, duration, and number (from both sensation and reflection). These are said to be "our original ideas," and the rest to be "derived" form or to "depend" on them. It is difficult to compare the two lists, instance by instance; but one example may be taken. According to the first list, hard is a simple idea; according to the second list, solidity is the original (and therefore simple) idea, and hard will be derived from it and depend on it. It is clear that, in making the former list, Lock was trying to get back to the primary data of our individual experience; whereas, in the second list, he is rather thinking of the objective reality on which our experience depends and which, he assumes, it reveals. But he does not observe the difference. He seems to forget his view that the original of all knowledge is to be found in the particular, in something "simple and unmixed." Thus he says without hesitation, "If any one asks me, what this solidity is, I send him to the senses to inform him. Let him put a flint of a football between his hands, and then endeavour to join them, and he will know" (Bk. 2:4:4). But he will not know without going a long way beyond the simple idea. The simple ideas in the case are certain muscular and tactual sensations; and he interprets these by other means (including knowledge of external objects and his own organism) when he says that the flint or the football is solid.
His doctrine of modes is also affected by this same inattention of the fact that a simple idea must be really simple. Thus he holds that "space and extension" is a simple idea given both by sight and by touch (Bk. 2:4). One would expect, therefore, that the original and simple idea of space would be the particular patch seen at any moment or the particular "feel" of the exploring limb. But we are told that "each idea of any different distance, or space, is a simple mode" or the idea of space (Bk. 2:8:4). Here again the simple idea is generalized. He professes to begin with the mere particulars of external and internal sense, and to show how knowledge -- which is necessarily general -- is evolved from them. But, in doing so, he assumes a general or universal element as already given in the simple idea.
Having gone so far, he might almost have been expected to take a further step and treat the perceptions of particular things as modes of the simple idea substance. But this he does not do. Substance is an idea regarding which he was in earnest with his own fundamental theory (however, he was perplexed about the origin of the idea of "substance in general" as well as of the ideas of "particular sorts of substances; Bk. 2:23:2-3). He admits that substance is a complex idea; that is to say, it is formed by the mind's action out of simple ideas. Now, this idea of substance marks the difference between having sensations and perceiving things. Its importance, therefore, is clear; but there is no clearness in explaining it. We are told that there is a "supposed or confused idea of substance" to which are joined, for example, "the simple idea of a dull whitish colour, with certain degrees of weight, hardness, ductility and fusibility," and, as a result, "we have the idea of lead."
A difficulty might have been avoided if substance could have been interpreted as simply the combination by the understanding of white, hard, etc., or some similar cluster of ideas of sensation. But it was not Locke's way thus to ignore facts. He sees that something more is needed than these ideas of sensation. They are only joined to "the supposed or confused idea of substance," which is there and "always the first and chief" (Bk. 2:12:6). He holds to it that the idea is a complex idea and so mad by the mind; but he is entirely at a loss to account for the materials out of which it is made. We cannot imagine how simple ideas can subsist by themselves, and so "we accustom ourselves to suppose some substratum wherein they do subsist," and this we call substance. In one place, he even vacillates between the assertions that we have no clear idea of substance and that we have no idea of it at all " (Bk. 1:3:19). It is "a supposition of he knows not what." This uncertainty, as will appear presently, throws its shadow over our whole knowledge of nature.