If we cannot recall at pleasure a single idea, we are not less unable to recall a train. Every person knows how evanescent his thoughts are, and how impossible it is for him to begin at the beginning of a past train, if it is not a train of the individual objects familiar to his senses, and go on to the end, neither leaving out any of the items which composed it, nor allowing any which did not belong to it, to enter in.
2. It is most obvious that, by ideas alone, the events which are passed, are to us any thing. If the objects which we have seen, heard, smelt, tasted, and touched, left no traces of themselves; if the immediate sensation were every thing, and a blank ensued when the sensation ended, the past would be to us as if it had never been. Yesterday would be as unknown as the months we passed in the womb, or the myriads of years before we were born.
3. It is only by our ideas of the past, that we have any power of anticipating the future. And if we had no power of anticipating the future, we should have no principle of action, but the physical impulses, which we have in common with the brutes. This great law of our nature, the anticipation of the future from the { 131} past, will be fully illustrated in a subsequent part of this inquiry: at present, all that is required is, the admission, which will probably not be refused, of this general truth: That the order, in which events have been observed to take place, is the order in which they are expected to take place; that the order in which they have taken place is testified to us only by our ideas; and that upon the correctness, with which they are so testified, depends the faculty we possess of converting the powers of nature into the instruments of our will; and of bringing to pass the events which we desire.
4. But all this power depends upon the order of our ideas. The importance, therefore, is unspeakable, of being able to insure the order of our ideas; to make, in other words, the order of a train of ideas correspond unerringly with a train of past sensations. We have not, however, a direct command over the train of our ideas. A train of ideas may have passed in our minds corresponding to events of great importance; but that train will not pass again, unvaried, except in very simple cases, without the use of _expedients_.