IMPRESSION. — The term acquires a technical meaning in those theories of knowledge that study the relationship between the external world and sensory experience, beginning especially with Aristotle, who compared the influence of the object on the senses to that of a seal on soft wax (cf.: *De anima*, II, 12, 424 a 19; the impression is called τὸ σημεῖον).
It is necessary to arrive at English empiricism to have a theory of the impression, particularly with D. Hume, who bases his radical phenomenism on the distinction between impression and idea (*Treatise on Human Nature*, I. II, sect. I; ed. L. A. Selby-Bigge, Oxford 1928, p. 1 ff.). Impressions are of two kinds: “impressions of sensation” if they come from the external world, and “impressions of reflection” if they arise from the reiteration of experiences already had, namely memories and ideas of the imagination. The distinction between impressions and ideas lies in the greater vividness and the character of conviction of reality that accompanies impressions, unlike ideas.