answer to it which he thinks true, till by hearing his reasons they were convinced.’ Mr. Locke then adds other instances to the same effect, as ‘That a man who reads or hears with attention and understanding takes little notice of the characters or sounds, but of the ideas that are excited in him by them. How frequently do we in a day cover our eyes with our eyelids, without at all perceiving that we are in the dark! Men that by custom have got the use of a by-word do almost in every sentence pronounce sounds, which though taken notice of by others, they themselves neither hear nor observe: and therefore it is not so strange that our mind should often change the idea of its sensation into that of its judgment, and make one serve only to excite the other without our taking notice of it.’ On the problem above stated, which has been often made a subject of dispute, I shall only remark that the answer given to it, with which Mr. Locke agrees, is directly repugnant to his doctrine of the real existence of the primary qualities of matter, namely figure and extension. For it is plain, that if there is any thing in external objects answering to their ideas in our minds, the ideas we have of those qualities and which are conveyed by different senses, must be like one another. If the ideas of figure as a visible and tangible thing have no resemblance to themselves, it is ridiculous to suppose that they can coincide with any thing out of them in nature. Secondly, it appears to me that the mind must recognise a certain similarity between the impressions of different senses in this case. For instance, the sudden change or discontinuity of the sensation, produced by the sharp angles of the cube, is something common to both ideas, and if so, must afford a means of comparing them together. Berkeley, in his ‘Essay on Vision,’ goes so far as to deny that there is any intuitive analogy between the ideas of number as conveyed by different senses, and asserts that the distinction between the two legs of a statue, for instance, as perceived by the touch or by the sight would not imply any idea of like or same. I grant this consequence to be true, on the principle maintained by him that there are no abstract ideas in the mind, for on this principle there can be no idea answering to the words _same_ or _different_, but then this argument would destroy all kind of coincidence not only between ideas of different senses, but between repeated impressions of the same sense. The ‘Essay on Vision,’ of which I have already spoken, apparently originated in the problem here inserted, and is a more complete exemplification of the effects of association with respect to objects of sight than is to be found even in Hartley’s account of this subject. Mr. Locke’s account of the distinction between wit and understanding I have already noticed; his explanation of the difference between idiots and madmen has been often referred to, and is as follows: