The whole of this subject is ably and exhaustively treated by Luys in "The Brain and Its Functions," but I have not room here to go into it fully. Briefly put, the scientific explanation may be expressed thus: The brain cells have power to receive and to _repeat_ impressions. When a new sensory impulse arises it awakens these impressions by means of the fibres of association. It is as though the brain were a phonographic "record." Upon this "record" there is printed, let us say, some moral lesson, as "Look not upon the wine when it is red in the cup." On the word "wine" being heard the association fibre which links the idea of wine to the moral idea of temperance sets the "record" in motion, and memory recalls the caution, "Look not upon the wine in the cup." It is as if a "record" on which is printed a song by Dan Leno were joined up with a battery which, upon hearing the word "Leno," would start the "record" to repeat the song. I hope I have made that clear. I will now conclude by quoting from Dr. Saleeby a passage dealing with the important subject of "association." I take it from "The Cycle of Life": Nerve cells are significantly incapable of division and reproduction.... All the experience of living merely modifies, the state of the cells already present. The modification is memory. But though a nerve-cell cannot divide, it can send forth new processes, or nerve-fibres from itself--what we call a nerve being simply a collection of processes from a nerve-cell. Throughout the brain and spinal cord we find great numbers of nerve processes which simply run from one set of nerve-cells to another, instead of running to a sense-organ, or a muscle, or a gland. Such fibres are called _association_ fibres, their business being to associate different sets of nerve-cells. It is conceivable that an exceptional development of such fibres may account for the possession of a good memory, or, at any rate, for the power easily to learn such co-ordinations as are implied in violin-playing, billiards, cricket, or baseball. Granting the power of nerve-cells, even when adult, to form new processes, it might be supposed that the exercise of this power accounts for the acquirement of certain habits of thought or action. Now, whether or not nerve-cells have power to form new association fibres late in life, it is important to notice that the association fibres which exist at birth or form in childhood are the means by which one idea suggests another; and the means by which, as I said just now, upon the utterance of the word "wine" all we have remembered to have read or heard about wine is repeated by the memory "record." And, just as a phonograph record can only repeat the song or speech that is printed upon it, so the memory can only repeat what it contains, and it contains nothing that has not been printed there through the medium of the senses.