  Diagrams along the lines of 6.1 are commonplace in the Retentionalist literature, and all such figures serve the useful purpose of making it clear that Retentionalist are offering what is, in effect, a two-dimensional model of temporal consciousness. The change and succession which feature in our immediate experience occur within specious presents which have no temporal extension in ordinary objective time; these specious presents do possess a temporal extension – an experienced extension – but this exists (as it were) orthogonally to ordinary time, along the vertical rather than the horizontal. Opting for a two-dimensional temporal framework is an elegant solution to the problem of how it is possible for succession and persistence to exist in experiences which (objectively) have no duration, but is also a source of potential difficulties. Since Extensional models do not require the positing of this additional dimension, Retentional models are vulnerable to the accusation that they are needlessly profligate.