Hume divided the representations that inhabit the mind into two types: impressions and ideas. Impressions are the basic building blocks of cognition. In contrast with ideas of memory and imagination, impressions come upon us with “force and liveliness.” Once we've had an impression, we often find that “it again makes its appearance there as an idea.” When this happens, the impression retains some of its vivacity and forcefulness, and “is somewhat intermediate betwixt an impression and an idea” [Hume, 1739, Bk 1, Part 1, Section 2], in which case it is a memory image.  Should it lose its vivacity entirely, it is then a “perfect idea” and is of the imagination.