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_. 5. The difference between the occasions of our IDEAS, and the occasions of our SENSATIONS, affords a resource for this purpose. Over the occasions of our sensations; we have an extensive power. We can command the smell of a rose, the hearing of a bell, the sight of a tree, the sensation of heat or of cold, and so on. Over the occasions of our ideas we have little or no direct power. Our ideas come and go. There is a perpetual train of them, one succeeding another; but we cannot will any link in that chain of ideas; each link is determined by the foregoing; and every man knows, how impossible {132} it is, by mere willing, to make such a train as he desires. Thoughts obtrude themselves without his bidding; and thoughts which he is in quest of will not arise.