All the best authorities recognize and teach the importance of this law of association, in connection with the memory. Abercombie says: "Next to the effect of attention is the remarkable influence produced upon memory by association." Carpenter says: "The recording power of memory mainly depends upon the degree of attention we give to the idea to be remembered. The reproducing power again altogether depends upon the nature of the associations by which the new idea has been linked on to other ideas which have been previously recorded.'' Ribot says: "The most fundamental law which regulates psychological phenomena is the law of association. In its comprehensive character it is comparable to the law of attraction in the physical world." Mill says: "That which the law of gravitation is to astronomy; that which the elementary properties of the tissues are to physiology; the law of association of ideas is to psychology." Stewart says: ''The connection between memory and the association of ideas is so striking that it has been supposed by some that the whole of the phenomena might be resolved into this principle. The association of ideas connects our various thoughts with each other, so as to present them to the mind in a certain order; but it presupposes the existence of those thoughts in the mind,—in other words it presupposes a faculty of retaining the knowledge which we acquire. On the other hand, it is evident that without the associating principle, the power of retaining our thoughts, and of recognizing them when they occur to us, would have been of little use; for the most important articles of our knowledge might have remained latent in the mind, even when those occasions presented themselves to which they were immediately applicable."

Association of ideas depends upon two principles known, respectively, as (1) the law of contiguity; and (2) the law of similarity. Association by contiguity is that form of association by which an idea is linked, connected, or associated with the sensation, thought, or idea immediately preceding it, and that which directly follows it. Each idea, or thought, is a link in a great chain of thought being connected with the preceding link and the succeeding link. Association by similarity is that form of association by which an idea, thought, or sensation is linked, connected, or associated with ideas, thoughts, or sensations of a similar kind, which have occurred previously or subsequently. The first form of association is the relation of sequence—the second the relation of kind.