David Hume, answered from the texts and cited to the page.
All the perceptions of the human mind resolve themselves into two distinct kinds.1 The first, which I call impressions, enter with most force and violence; under this name I comprehend all our sensations, passions, and emotions as they make their first appearance in the soul.1 The second, which I call ideas, are the faint images of these impressions in thinking and reasoning — the copies, as it were, that the mind retains after the vivid originals have subsided.1
The distinction is one every person will readily perceive from their own experience: feeling and thinking differ, and that difference is the difference between impression and idea. Beyond this primary division, each kind admits of a further distinction into simple and complex. Simple perceptions are such as admit of no distinction nor separation; complex ones may be distinguished into parts.2
An apple presents colour, taste, and smell united together, yet these are plainly distinguishable qualities — a ready illustration of complexity.2 Now the critical question is how impressions and ideas stand in relation to one another, which of them is cause and which effect. The proposition I establish in the Treatise of Human Nature — my first systematic work, in which I laid out the whole architecture of the understanding — is that all our simple ideas in their first appearance are derived from simple impressions which correspond to them and which they exactly represent.3
The order of appearance settles the matter: simple impressions always take precedence over their correspondent ideas, never the contrary.3 To give a child an idea of scarlet, I present the object and convey the impression; I do not proceed so absurdly as to attempt producing the impression by exciting the idea.3 This priority of impressions is the ground of what I call the copy principle, and it does genuine philosophical work.
In the Enquiry Concerning Human Understanding, where I returned to these questions with a clearer and more accessible hand, I pressed the principle against the obscure terms of metaphysics — power, force, energy, necessary connexion.4 When such terms breed ambiguity, the remedy is to produce the impression from which the idea is copied; if no impression can be found, the idea has no determinate meaning, and the term that purports to express it is suspect.4
It is, in effect, a new microscope for the moral sciences: the impression, being strong and sensible, throws light on the correspondent idea that would otherwise lie in obscurity.4
Those perceptions, which enter with most force and violence, we may name impressions; and under this name I comprehend all our sensations, passions and emotions, as they make their first appearance in the soul. By ideas I mean the faint images of these in thinking and reasoning.A Treatise of Human Nature, p. 7
Simple perceptions or impressions and ideas are such as admit of no distinction nor separation. The complex are the contrary to these, and may be distinguish'd into parts. Tho' a particular colour, taste, and smell are qualities all united together in this apple, 'tis easy to perceive they are not the same, but are at least distinguishable from each other.A Treatise of Human Nature, pp. 7–8
all our simple ideas in their first appearance are deriv'd from simple impressions, which are correspondent to them, and which they exactly represent... the simple impressions always take the precedence of their correspondent ideas, but never appear in the contrary order. 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.A Treatise of Human Nature, p. 9
all our ideas are nothing but copies of our impressions, or, in other words, that it is impossible for us to think of any thing, which we have not antecedently felt... Produce the impressions or original sentiments, from which the ideas are copied. These impressions are all strong and sensible. They admit not of ambiguity.An Enquiry Concerning Human Understanding, p. 22