§ 2. Association of Ideas. — For a general account of the nature of Association, we must refer to bk. i., ch. ii., § 7 Association and Reproduction § 9 The Various Modes of Specific Reproduction. The basis of all associative connexion is the concurrence of distinct experiences in the formation of a single cumulative disposition, which tends to be reexcited as a whole whenever any of the experiences recur which have combined to produce it. If we suppose that two experiences, a and b, have been united in this way so as to form the total disposition Dab, the reoccurrence of an experience similar to a will reexcite Dab. If the reproduction takes the form of mere acquirement of meaning or of complication, the result is a modification of a, which we may represent by ab. But in ideal reproduction something more takes place. The occurrence of ab is followed by the ideal revival of ba, as a relatively independent phase in the successive flow of mental process.

It must be noted that the tendency is to the revival of the total experience ab. Hence, apart from interfering conditions, b will tend to be revived in the same relation to a as that in which it originally occurred. If in the original experience one object has been apprehended as succeeding another, or as situated on the top of another, or as logically dependent on another, the tendency of the ideal revival will be to represent the object in the same relations. It is evident that these objective relations may be indefinitely numerous and diverse in nature. Hence it is impossible to base on them a classification of the various forms of association of ideas. As Reid remarks: "Every relation of things has a tendency, more or less, to lead the thought, in a thinking mind, from one to the other."* It follows that in classifying the forms of association of ideas, we must consider relations between psychical states, as such, as distinguished from relations between the objects of which they take cognisance. Ultimately, all depends on continuity of interest: but this continuity may be direct or indirect, giving rise to two forms of association which are commonly called association by contiguity and by similarity.

* Works, Hamilton's edition, vol. i., p. 386.