All the best authorities recognize and teach the importance of this law of association, in connection with the memory. Abercrombie 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.
Association by contiguity is the great law of thought, as well as of memory. As Kay says: "The great law of mental association is that of contiguity, by means of which sensations and ideas that have been in the mind together or in close succession, tend to unite together, or cohere in such a way that the one can afterward recall the other. The connection that naturally subsists between a sensation or idea in the mind, and that which immediately preceded or followed it, is of the strongest and most intimate nature. The two, strictly speaking, are but one, forming one complete thought." As Taine says: "To speak correctly, there is no isolated or separate sensation. A sensation is a state which begins as a continuation of preceding ones, and ends by losing itself in those following it; it is by an arbitrary severing, and for the convenience of language, that we set it apart as we do; its beginning is the end of another, and its ending the beginning of another." As Ribot says: "When we read or hear a sentence, for example, at the commencement of the fifth word something of the fourth word still remains. Association by contiguity may be separated into two sub-classes—contiguity in time; and contiguity in space. In contiguity in time there is manifested the tendency of the memory to recall the impressions in the same order in which they were received—the first impression suggesting the second, and that the third, and so on. In this way the child learns to repeat the alphabet, and the adult the succeeding lines of a poem. As Priestly says: "In a poem, the end of each preceding word being connected with the beginning of the succeeding one, we can easily repeat them in that order, but we are not able to repeat them backwards till they have been frequently named in that order." Memory of words, or groups of words, depends upon this form of contigious association. Some persons are able to repeat long poems from beginning to end, with perfect ease, but are unable to repeat any particular sentence, or verse, without working down to it from the beginning. Contiguity in space is manifested in forms of recollection or remembrance by "position." Thus by remembering the things connected with the position of a particular thing, we are enabled to recall the thing itself. As we have seen in a preceding chapter, some forms of memory systems have been based on this law. If you will recall some house or room in which you have been, you will find that you will remember one object after another, in the order of the relative positions, or contiguity in space, or position. Beginning with the front hall, you may travel in memory from one room to another, recalling each with the objects it contains, according to the degree of attention you bestowed upon them originally. Kay says of association by contiguity: "It is on this principle of contiguity that mnemonical systems are constructed, as when what we wish to remember is associated in the mind with a certain object or locality, the ideas associated will at once come up; or when each word or idea is associated with the one immediately preceding it, so that when the one is recalled the other comes up along with it, and thus long lists of names or long passages of books can be readily learnt by heart."
From the foregoing, it will be seen that it is of great importance that we correlate our impressions with those preceding and following. The more closely knitted together our impressions are, the more closely will they cohere, and the greater will be the facility of remembering or recollecting them. We should endeavor to form our impressions of things so that they will be associated with other impressions, in time and space. Every other thing that is associated in the mind with a given thing, serves as a "loose end" of memory, which if once grasped and followed up will lead us to the thing we desire to recall to mind.
Association by similarity is the linking together of impressions of a similar kind, irrespective of time and place. Carpenter expresses it as follows: "The law of similarity expresses the general fact that any present state of consciousness tends to revive previous states which are similar to it.... Rational or philosophical association is when a fact or statement on which the attention is fixed is associated with some fact previously known, to which it has a relation, or with some subject which it is calculated to illustrate." And as Kay says: "The similars may be widely apart in space or in time, but they are brought together and associated through their resemblance to each other. Thus, a circumstance of to-day may recall circumstances of a similar nature that occurred perhaps at very different times, and they will become associated together in the mind, so that afterwards the presence of one will tend to recall the others." Abercrombie says of this phase of association: "The habit of correct association—that is, connecting facts in the mind according to their true relations, and to the manner in which they tend to illustrate each other, is one of the principle means of improving the memory, particularly that kind of memory which is an essential quality of a cultivated mind—namely, that which is founded not upon incidental connections, but on true and important relations."
As Beattie says: "The more relations or likenesses that we find or can establish between objects, the more easily will the view of one lead us to recollect the rest." And as Kay says: "In order to fix a thing in the memory, we must associate it with something in the mind already, and the more closely that which we wish to remember resembles that with which it is associated, the better is it fixed in the memory, and the more readily is it recalled. If the two strongly resemble each other, or are not to be distinguished from each other, then the association is of the strongest kind.... The memory is able to retain and replace a vastly greater number of ideas, if they are associated or arranged on some principle of similarity, than if they are presented merely as isolated facts. It is not by the multitude of ideas, but the want of arrangement among them, that the memory is burdened and its powers weakened." As Arnott says: "The ignorant man may be said to have charged his hundred hooks of knowledge (to use a rude simile), with single objects, while the informed man makes each hook support a long chain to which thousands of kindred and useful things are attached."
We ask each student of this book to acquaint himself with the general idea of the working features of the law of association as given in this chapter for the reason that much of the instruction to be given under the head of the several phases and classes of memory is based upon an application of the Law of Association, in