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.
By the power, however, which we have over the occasions of our sensations, we can make sure of having a train of sensations exactly the same as we have had before. This affords us the means of having a train of ideas exactly the same as we have had before. If we choose a number of sensible objects, and make use of them as marks of our ideas, we can ensure any succession which we please of the sensible objects; and, by the association between them and the ideas, a corresponding succession of the ideas.
6. To one of the two sets of occasions, upon which Signs are thus useful, _evanescent_ Signs are the best adapted; _permanent_ signs are absolutely necessary for the other. For the purposes of speech, or immediate communication, sounds are the most convenient marks. Sounds, however, perish in the making. But for the purpose of retracing a train of ideas, which we have formerly had, it is necessary we should have marks which do not perish. Marks, addressed to the sight, or the touch, have the requisite permanence; and, of the two, those addressed to the eye have the advantage. Of marks addressed to the eye, two kinds have been adopted; either marks immediately of the ideas intended to be recalled; such as the picture-writing, or hieroglyphics, of some nations: or, visible marks, by letters, of the audible marks employed in oral communication. This latter kind has been found the most convenient, and in use among the largest, and most intelligent portion of our species.