All "perceptions," he says, are either "_Impressions_" or "_Ideas_." Under "impressions" he includes "all our more lively perceptions, when we hear, see, feel, love, or will;" in other words, "all our sensations, passions, and emotions, as they make their first appearance in the soul" (I.p.

15).
"Ideas," on the other hand, are the faint images of impressions in thinking and reasoning, or of antecedent ideas.
Both impressions and ideas may be either _simple_, when they are incapable of further analysis, or _complex_, when they may be resolved into simpler constituents.

All simple ideas are exact copies of impressions; but, in complex ideas, the arrangement of simple constituents may be different from that of the impressions of which those simple ideas are copies.
Thus the colours red and blue and the odour of a rose, are simple impressions; while the ideas of blue, of red, and of rose-odour are simple copies of these impressions.

But a red rose gives us a complex impression, capable of resolution into the simple impressions of red colour, rose-scent, and numerous others; and we may have a complex idea, which is an accurate, though faint, copy of this complex impression.
Once in possession of the ideas of a red rose and of the colour blue, we may, in imagination, substitute blue for red; and thus obtain a complex idea of a blue rose, which is not an actual copy of any complex impression, though all its elements are such copies.
Hume has been criticised for making the distinction of impressions and ideas to depend upon their relative strength or vivacity.

Yet it would be hard to point out any other character by which the things signified can be distinguished.