Of this remarkable case of association, that which we call "Our Belief in External Objects" is one of the most remarkable instances. Of the sensations, of sight, of handling, of smell, of taste, which I have from a rose, each is an event; with each of those events, I associate the idea of a constant antecedent, a cause; that cause unknown, but furnished with a name, by which it may be spoken of, namely, quality; the quality of red, the cause of the sensation red; the qualities of consistence, extension and figure, the causes of the sensations of handling; the qualities of smell and taste, the causes of the sensations of smell and taste. Such is one part of the process of association in this case. Another is that by which the ideas of those sensations are so intimately united, as to appear not several ideas, but one idea, the idea of a rose. We have now two steps of association; that {353} of the several sensations into one idea; that of the several sensations each with a separate cause. But we do not stop here; for, as in a train of events, consisting of several links, A, B, C, D, and so on, though C is the antecedent or cause of D, it is itself the consequent or effect of B; and in all cases, when we have found the cause of any particular event, we have still to find out what was the cause of that cause. In this manner, when our habit of association has carried us from our sensations to the causes of them, the same habit carries us still farther. As each of our sensations must have a cause, to which, as unknown, we give the name quality; so each of those qualities must have a cause. And as the ideas of a number of sensations, concomitant in a certain way, are combined into a single idea; as that of rose, that of apple; the unity, which is thus given to the effects, is of course transferred to the supposed causes, called qualities: they are referred to a common cause. To this supposed cause of supposed causes, we give a name; and that name is the word _Substratum_. It is obvious, that there is no reason for stopping at this _Substratum_; for, as the sensation suggested the quality, the quality the substratum, the substratum as properly leads to another antecedent, another substratum, and so on, from substratum to substratum, without end. These inseparable associations, however, rarely go beyond a single step, hardly ever beyond two. The Barbarian, in accounting for the support of the earth, placed it on the back of a great elephant, and the great elephant on the back of a great tortoise; but neither himself, nor those whom he {354} instructed, were carried by their habits of association any farther.[100]