In discussing the idea of Substance, Locke seems generally to be thinking more of Matter than Mind. But, in an early part of the _Essay_ (Bk. II., ch. xiii., § 18), he very rightly begs those who talk so much of Substance "to consider whether applying it, as they do, to the infinite incomprehensible God, to finite Spirit, and to Body, it be in the same sense, and whether it stands for the same idea, when each of those three so different beings are called Substances." As applied respectively to Matter and to Mind (whether finite or infinite), it appears to me that the word Substance assumes a very different meaning, and that the absurdities which it is possible to fix on the distinction between Matter and its attributes by no means extend to the distinction between Mind and its operations. For an union of certain forces or powers affecting our organisms in certain ways seems to exhaust our conception of external objects (the notion of externality, I conceive, being quite independent of that of the Substrate "matter"), but no similar enumeration of mental acts and feelings seems adequately to take the place of that "Self," or "I," of which we regard these as merely phases and modifications. It would much conduce to clearness in philosophical discussions if, at least amongst those who admit the dualism of matter and mind, the word Substance, whenever applied to incorporeal objects, were replaced by the word Mind, and, whenever applied to corporeal objects, by the word Matter. * * * * * The Second Book closes, in the Fourth and subsequent editions, with a short but very interesting Chapter on the "Association of Ideas." The student of Mental Philosophy will find it instructive to compare this Chapter with the previous account given by Hobbes (_Human Nature_, ch. iv.; _Leviathan_, Pt. I., ch. iii.), and the subsequent account given by Hume (_Human Nature_, Pt. I., § 4; _Essays on Human Understanding_, § 3), of the same phenomena. Locke appears to have been the first author to use the exact[2] expression "Association of Ideas," and it is curious to find in this chapter (§ 5) the word "inseparable," so familiar to the readers of recent works on psychology, already applied to designate certain kinds of association. Some ideas, indeed, have, he says, a natural correspondence, but others, that "in themselves are not at all of kin," "come to be so united in some men's minds that one no sooner at any time comes into the understanding than the whole Gang, always inseparable, show themselves together." [2] Sir W. Hamilton refers to La Chambre (_Système de l'Ame_: Paris, 1664) as having anticipated Locke in the use of this expression. In Liv. IV., ch. ii., art. 9, La Chambre speaks of "l' Union et la Liaison des Images," but I cannot find that he approaches any nearer to the now established phraseology.