{ Sensations or outer { Impressions { impressions { (= original) { Feelings or inner { { impressions Perceptions { (= mental { { Memories or an exact states) { { reproduction of an { { impression or of a { Ideas { combination of impressions { (= derived) { Imagination or a combination, { separation, and transposition { of impressions according to { the imagination’s own laws. It should be noted, however, that the above classes are not coördinate according to Hume. Impressions are prior to ideas, and of the impressions the feelings or inner impressions are “posterior to the sensations and derived from them.” Hume is a sensationalist, for the most original of the impressions are sensations. =The Association of Ideas.= Since nothing can enter the mind except through the two portals of outer and inner impressions, every idea in the mind is the copy of one or several impressions. How then can there be any such thing as error? Error arises from the understanding and imagination in their manipulation of the impressions――from the faculties of the mind combining, separating, and transposing the impressions and their memories. An idea resulting from such transposition may and often is referred to an impression different from the one of which it is the copy. What does Hume mean by the faculties and powers of the mind? He does not mean that the mind with its functions exists as a reality, since all that exist are impressions and the copies of impressions or ideas. Hume means by mental faculties and powers the various modes by which ideas combine. Hume makes no distinction between memory, imagination, judgment, conception, etc., except (1) as different groupings of ideas and (2) as accompanied by different feelings. _The whole mental life and the faculties of the mental life are nothing but an association of ideas._ Isolated ideas are explained as copies of isolated impressions; and from these ideas are derived groups of ideas which we call trains of thought. Why do ideas group themselves together? The only answer is that it is the nature of ideas. Hume frequently speaks of these associative relations as “the manner of conceiving ideas.” He also says that there is a “gentle force” or “determination” of the ideas to relate themselves with other ideas. Given the impressions and their relations, and Hume will explain the whole knowing process. Associative relations take an important place in Hume’s theory, but some critics say that they are interlopers; that he has introduced them by a back door; that they are not mentioned in his psychological inventory.