We find, by experience, that when any impression has been present with the mind, it again makes its appearance there as an idea; and this it may do after two different ways: either when, in its new appearance, it retains a considerable degree of its first vivacity, and is somewhat intermediate betwixt an impression and an idea; or when it entirely loses that vivacity, and is a perfect idea. The faculty by which we repeat our impressions in the first manner, is called the _memory_, and the other the _imagination_. 'Tis evident, at first sight, that the ideas of the memory are much more lively and strong than those of the imagination, and that the former faculty paints its objects in more distinct colours, than any which are employed by the latter. When we remember any past event, the idea of it flows in upon the mind in a forcible manner; whereas, in the imagination, the perception is faint and languid, and cannot, without difficulty, be preserved by the mind steady and uniform for any considerable time. Here, then, is a sensible difference betwixt one species of ideas and another. But of this more fully hereafter.[2] There is another difference betwixt these two kinds of ideas, which is no less evident, namely, that though neither the ideas of the memory nor imagination, neither the lively nor faint ideas, can make their appearance in the mind, unless their correspondent impressions have gone before to prepare the way for them, yet the imagination is not restrained to the same order and form with the original impressions; while the memory is in a manner tied down in that respect, without any power of variation. 'Tis evident, that the memory preserves the original form, in which its objects were presented, and that wherever we depart from it in recollecting any thing, it proceeds from some defect or imperfection in that faculty. An historian may, perhaps, for the more convenient carrying on of his narration, relate an event before another to which it was in fact posterior; but then, he takes notice of this disorder, if he be exact; and, by that means, replaces the idea in its due position. 'Tis the same case in our recollection of those places and persons, with which we were formerly acquainted. The chief exercise of the memory is not to preserve the simple ideas, but their order and position. In short, this principle is supported by such a number of common and vulgar phenomena, that we may spare ourselves the trouble of insisting on it any farther.