  The dependency of Diodorus' modal notions on the propositions’ truth-values-at-times implies that some propositions can change their modality from possible to impossible and from non-necessary to necessary. For instance, if we assume that the proposition ‘Artemisia is five years old’ is now true, then that proposition is now possible; but it will presumably be impossible at some future time, namely once Artemisia has reached the age of six, since from then on it will never be true again (assuming linearity of time).