If you need any proof of the importance of association by relation, and of the laws governing its action, you have but to recall the ordinary "train of thought" or "chain of images" in the mind, of which we become conscious when we are day-dreaming or indulging in reverie, or even in general thought regarding any subject. You will see that every mental image or idea, or recollection is associated with and connected to the preceding thought and the one following it. It is a chain that is endless, until something breaks into the subject from outside. A fact flashes into your mind, apparently from space and without any reference to anything else. In such cases you will find that it occurs either because you had previously set your subconscious mentality at work upon some problem, or bit of recollection, and the flash was the belated and delayed result; or else that the fact came into your mind because of its association with some other fact, which in turn came from a precedent one, and so on. You hear a distant railroad whistle and you think of a train; then of a journey; then of some distant place; then of some one in that place; then of some event in the life of that person; then of a similar event in the life of another person; then of that other person; then of his or her brother; then of that brother's last business venture; then of that business; then of some other business resembling it; then of some people in that other business; then of their dealings with a man you know; then of the fact that another man of a similar name to the last man owes you some money; then of your determination to get that money; then you make a memorandum to place the claim in the hands of a lawyer to see whether it cannot be collected now, although the man was "execution proof" last year—from distant locomotive whistle to the possible collection of the account. And yet, the links forgotten, the man will say that he "just happened to think of" the debtor, or that "it somehow flashed right into my mind," etc. But it was nothing but the law of association—that's all. Moreover, you will now find that whenever you hear mentioned the term "association of mental ideas," etc., you will remember the above illustration or part of it. We have forged a new link in the chain of association for you, and years from now it will appear in your thoughts.