David Hume, answered from the texts and cited to the page.
The first principle I establish in the science of human nature is this: all our simple ideas, in their first appearance, are derived from simple impressions, which correspond to them and which they exactly represent.1 The argument for this rests on a straightforward empirical observation. When I examine the order of appearance between impressions and ideas, I find by constant experience that simple impressions always take precedence — they come first, and their correspondent ideas follow.2
To give a child an idea of scarlet, I present the object; I do not attempt the absurdity of producing the impression by exciting the idea.3 Our ideas, on their appearance, produce not their correspondent impressions; but any impression of mind or body is constantly followed by an idea that resembles it, differing only in force and liveliness.
This constant conjunction, in so great a number of instances, cannot arise from chance, and clearly proves that impressions are the causes of ideas, not ideas of impressions.4 The principle has an immediate philosophical use. When any term in metaphysics grows obscure — power, force, energy, necessary connexion — the remedy is to demand the impression from which the correspondent idea is copied.5
Impressions are strong and sensible; they admit not of ambiguity. By tracing an idea back to its original impression, we place it in a full light, and what seemed a deep mystery either becomes clear or reveals itself as no genuine idea at all. I have called this a new microscope for the moral sciences: the most minute ideas, enlarged by this method, fall readily under our apprehension.6
One qualification deserves notice. Ideas can produce images of themselves in new ideas — memory and imagination work precisely this way — but since those first ideas were themselves derived from impressions, it remains true that all our simple ideas proceed, either mediately or immediately, from their correspondent impressions.7 The chain of derivation may be long, but it terminates in sensation or reflection, never in pure thought alone.
This, finally, is the same question that has made so much noise under the heading of innate ideas. Philosophers who deny that ideas of extension or passion are innate do nothing but show that they are conveyed by the senses or by prior experience of the emotions themselves — which is only to say that ideas are preceded by more lively perceptions from which they are derived.8
The copy principle states that conclusion precisely, and I trust its clear statement will render it of more use in our reasonings than the older, less exact vocabulary permitted.
all our simple ideas in their first appearance are deriv'd from simple impressions, which are correspondent to them, and which they exactly represent.A Treatise of Human Nature, p. 9
I consider the order of their first appearance; and find by constant experience, that the simple impressions always take the precedence of their correspondent ideas, but never appear in the contrary order.A Treatise of Human Nature, p. 9
To give a child an idea of scarlet or orange, of sweet or bitter, I present the objects, or in other words, convey to him these impressions; but proceed not so absurdly, as to endeavour to produce the impressions by exciting the ideas.A Treatise of Human Nature, p. 9
The constant conjunction of our resembling perceptions, is a convincing proof, that the one are the causes of the other; and this priority of the impressions is an equal proof, that our impressions are the causes of our ideas, not our ideas of our impressions.A Treatise of Human Nature, p. 9
To be fully acquainted, therefore, with the idea of power or necessary connexion, let us examine its impression; and in order to find the impression with greater certainty, let us search for it in all the sources, from which it may possibly be derived.An Enquiry Concerning Human Understanding, p. 22
by this means, we may, perhaps, attain a new microscope or species of optics, by which, in the moral sciences, the most minute, and most simple ideas may be so enlarged as to fall readily under our apprehension.An Enquiry Concerning Human Understanding, p. 22
Ideas produce the images of themselves in new ideas; but as the first ideas are suppos'd to be deriv'd from impressions, it still remains true, that all our simple ideas proceed, either mediately or immediately, from their correspondent impressions.A Treatise of Human Nature, pp. 10–11
if we carefully examine these arguments, we shall find that they prove nothing but that ideas are preceded by other more lively perceptions, from which they are deriv'd, and which they represent.A Treatise of Human Nature, pp. 10–11