  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 case there is the immediately perceived motion and noise: sharp, vivid – LOUD – and out there. Between these two forms of experience there is all the difference in the world. Since it is not plausible to suppose that ordinary memory-images are responsible for our immediate experience of change, retentions must be a of a distinctive (and not commonly recognised) form of experience.