Retentions re-present the past, as memories do, but must be more vivid than ordinary memories to fulfil their role in grounding immediate experience of change.

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