There is another set of cases, very familiar, but affording very important evidence on the subject. It frequently happens, that there are matters which we desire not to forget. What is the contrivance to which we have recourse for preserving the memory; that is, for making sure that it will be called into existence, when it is our wish that it should. All men, invariably employ the same expedient. They endeavour to form an association between the idea of the thing to be remembered, and some sensation, or some idea, which they know beforehand will occur at or near the time when they wish the remembrance to be in their minds. If this association is formed, and the sensation or the idea, with which it has been formed, occurs; the sensation, or idea, calls up the remembrance; and the object of him who formed the association is attained. To use a vulgar instance; a man receives a commission from his friend, and, that he may not forget it, ties a knot on his handkerchief. How is this fact to be explained? First of all, the idea of the commission is associated with the making of the knot. Next, the handkerchief is a thing which it is known beforehand will be frequently seen, and of {324} course at no great distance of time from the occasion on which the memory is desired. The handkerchief being seen, the knot is seen, and this sensation recalls the idea of the commission, between which and itself, the association had been purposely formed. What is thus effected through association with a sensation, may be effected through association with an idea. If there is any idea, which I know will occur to me at a particular time, I may render myself as sure of recalling any thing which I wish to remember at that time, by associating it with this idea, as if I associated it with a sensation. Suppose I know that the idea of Socrates will be present to my mind at twelve o'clock this day week: if I wish to remember at that time something which I have to do, my purpose will be gained, if I establish between the idea of Socrates, and the circumstance which I wish to remember, such an association that the one will call up the other.