Section II
I took notice in the Introduction, that those ideas which resemble sensations were called ideas of sensation; and also that they might be called simple ideas, in respect of the intellectual ones which are formed from them, and of whose very essence it is to be complex. But the ideas of sensation are not entirely simple, since they must consist of parts both co-existent and successive, as the generating sensations themselves do.
Now, that the simple ideas of sensation are thus generated, agreeably to the proposition, appears, because the most vivid of these ideas are those where the corresponding sensations are most vigorously impressed, or most frequently renewed; whereas, if the sensation be faint, or uncommon, the generated idea is also faint in proportion, and, in extreme cases, evanescent and imperceptible. The exact observance of the order of place in visible ideas, and of the order of time in audible ones, may likewise serve to shew, that these ideas are copies and offsprings of the impressions made on the eye and ear, in which the same orders were observed respectively. And though it happens, that trains of visible and audible ideas are presented in sallies of the fancy, and in dreams, in which the order of time and place is different from that of any former impressions, yet the small component parts of these trains are copies of former impressions; and reasons may be given for the varieties of their compositions.
It is also to be observed, that this proposition bears a great resemblance to the third; and that, by this resemblance, they somewhat confirm and illustrate one another. According to the third proposition, sensations remain for a short time after the impression is removed; and these remaining sensations grow feebler and feebler, till they vanish. They are therefore, in some part of their declension, of about the same strength with ideas, and in their first state, are intermediate between sensations and ideas. And it seems reasonable to expect, that, if a single sensation can leave a perceptible effect, trace, or vestige, for a short time, a sufficient repetition of a sensation may leave a perceptible effect of the same kind, but of a more permanent nature, i.e. an idea, which shall recur occasionally, at long distances of time, from the impression of the corresponding sensation, and vice versâ. As to the occasions and causes, which
