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 Eibot 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.