Hume's Theory of Impressions and Ideas

David Hume (1711–1776) opens Book I of A Treatise of Human Nature (published anonymously in three volumes, 1739–1740, when Hume was still in his twenties) with a foundational distinction between two kinds of "perceptions of the mind": impressions and ideas. Impressions, for Hume, comprise all our more lively and forceful perceptions — sensations (seeing a color, feeling a pain, tasting a flavor), together with passions and emotions as they are first felt (anger, love, desire) — while ideas are the fainter, less forceful copies of impressions that occur in thinking, remembering, and imagining, such as recalling the color one saw or imagining a pain one has not currently felt. This distinction of degree, rather than of fundamental kind, is refined further into simple and complex perceptions: simple impressions and ideas admit of no distinction or separation into parts (the specific shade of red one sees is a simple impression), while complex ones, such as the idea of a particular city or the idea of a golden mountain, are composed of simple parts combined by the imagination.