The association of ideas has been treated of at large by several philosophers ; and it makes a conspicuous figure in various philosophical systems, particularly those of Hartley and Hume. The first of these endea vours to explain its origin mechanically, by means of the coincident vibrations or vibratiunries of the nerves. Particular sensations he supposes to result from previous vibrations conveyed through the nerves to the medulla ry substance of the brain ; which may be so intimately associated together, that one of them, whet, impressed alone, shall be able to excite in the mind the ideas of all the rest. He distinguishes associations into synchronous and successive ; and ascribes all our complex ideas to the influence of this principle. It is by a sy echronous association, that the sight of a part of a large building suggests the idea of the rest instantaneously ; and by a successive association, that the sound of the words which begin a familiar sentence, brings to remembrance the remaining parts in their order. Dr Hartley appre hends, that by pursuing and perfecting his doctrine of association, we may some time or other be enabled to analyse those complex ideas that are commonly called the ideas of reflection, or intellectual ideas, into their several component parts, i. e. into the simple ideas of sensation of which they consist ; and thus illustrate and explain the various phenomena of the human mind. See Observations on Man, part i.
Mr Hume, like Dr Hartley, ascribes great influence to the principle of association, in accounting for the va rious operations of intellect. He remark:, that ideas are never introduced into the mind, but in consequence of an order previously established among them by asso ciation, which may be traced not only in our more se rious thinking and conversation, but in our lowest dis course, and most wandering reveries, and even in our dreams. Among different languages, he remarks also, words expressive of ideas the most compounded nearly correspond to each other ; and hence it may be inferred, that the simple ideas comprehended in the compound.
ones, are bound together by some universal principle, which has an equal influence on all mankind. Accord ing to Mr Hume, there are three principles, by which the connexion, or association, among our ideas is regu lated. These are, resemblance, contiguity in time or place, and cause and effect. These, he says, are the only bonds that unite our thoughts together, and beget that regular train of reflection or discourse, which, in a greater or less degree, takes place among all mankind.
Essays, vol. i. p. 56.
This enumeration of the principles which regulate the association of ideas is very defective, and several other causes of association have been added to it by succeeding writers ; such as analogy, contrast, mutual dependence, and custom or habit. Contrast, or contra riety, is certainly a cause of association as well as re semblance ; for we are never more disposed to think of warmth than when we are cold, or of food than when we are hungry ; and youth and beauty not unfrequcntly suggest to our minds, the notions of decrepitude and de cay. With respect to habit, it may be doubted whether it deserves to be ranked as a separate principle of asso ciation ; or whether, indeed, it be any thing else than the principle of association itself exhibited in its most active state. By long practice we become so familiar with a certain succession of ideas, or with a certain se ries of associated thoughts and actions, that the one is suggested by the other, apparently without any effort of our own ; and the action follows the thought with which it is connected so readily, that we are unconscious of any effort of volition previous to the active exertion ; and this it is that in most cases constitutes habit. See