[SECTION II.]
DIVISION OF THE SUBJECT.
Since it appears, that our simple impressions are prior to their correspondent ideas, and that the exceptions are very rare, method seems to require we should examine our impressions before we consider our ideas. Impressions may be divided into two kinds, those of sensation, and those of reflection. The first kind arises in the soul originally, from unknown causes. The second is derived, in a great measure, from our ideas, and that in the following order. An impression first strikes upon the senses, and makes us perceive heat or cold, thirst or hunger, pleasure or pain, of some kind or other. Of this impression there is a copy taken by the mind, which remains after the impression ceases; and this we call an idea. This idea of pleasure or pain, when it returns upon the soul, produces the new impressions of desire and aversion, hope and fear, which may properly be called impressions of reflection, because derived from it. These again are copied by the memory and imagination, and become ideas; which, perhaps, in their turn, give rise to other impressions and ideas: so that the impressions of reflection are only antecedent to their correspondent ideas, but posterior to those of sensation, and derived from them. The examination of our sensations belongs more to anatomists and natural philosophers than to moral; and, therefore, shall not at present be entered upon. And, as the impressions of reflection, viz. passions, desires, and emotions, which principally deserve our attention, arise mostly from ideas, 'twill be necessary to reverse that method, which at first sight seems most natural; and, in order to explain the nature and principles of the human mind, give a particular account of ideas, before we proceed to impressions. For this reason, I have here chosen to begin with ideas.
[SECTION III.]
OF THE IDEAS OF THE MEMORY AND IMAGINATION.
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.
The same evidence follows us in our second principle, of the liberty of the imagination to transpose and change its ideas. The fables we meet with in poems and romances put this entirely out of question. Nature there is totally confounded, and nothing mentioned but winged horses, fiery dragons, and monstrous giants. Nor will this liberty of the fancy appear strange, when we consider, that all our ideas are copied from our impressions, and that there are not any two impressions which are perfectly inseparable. Not to mention, that this is an evident consequence of the division of ideas into simple and complex. Wherever the imagination perceives a difference among ideas, it can easily produce a separation.