Ordinary memory-images of perceiving an event (e.g. recalling the sight and sound of a falling tree) are qualitatively distinct from the immediate perception of that event — lacking its sharpness, vividness, and externality.

Given the requirements of realism, what characteristics must retentions themselves possess? Retentions are akin to memories in that they re-present the past, but to fulfil the task required of them they must be more vivid than ordinary memories. We all know what it is like to call up a memory of seeing and hearing the falling of a tree; we all know what it is like to actually see and hear the falling of a tree. In the case of the former there is a visual and acoustic mental images. In the latter