IMPRESSION. - The term acquires a technical meaning in those theories of knowledge that study the relation between the external world and sensory experience, beginning especially with Aristotle, who compared the influence of the object upon the senses to that of a seal upon soft wax (cf.: De anima, II, 12, 424 a 19: i. is called τὸ σημεῖον).
One must come to English empiricism in order to find a theory of i., and especially to D. Hume, who bases his radical phenomenalism upon the distinction between i. and idea (Treatise on human nature, I. II, sect. I; ed. L. A. Selby-Bigge, Oxford 1928, p. 1 ff.). There are two kinds of i.: “i. of sensation” if they come from the external world, “i. of reflection” if they arise from the repetition of experiences already had, that is, from ideas of memory and imagination. The distinction between i. and ideas lies in the greater vividness and in the character of conviction of reality accompanying i., as distinct from ideas.