  Their treatment of conditionals and modalities implies that — like most Hellenistic philosophers — Diodorus and Philo worked with a concept of proposition that differs from ours in that it allows truth-values to change over time. For instance, the standard example for a simple proposition, ‘It is day’, does not contain a covert fixed date or definite time determination; rather, it ranges over times, changing its truth-value twice daily, all the while remaining the same proposition. Thus we may think of propositions as they were understood by Hellenistic philosophers as functions of time. Diodorus Cronus can perhaps also be credited with the beginnings of temporal logic, as both his theory of the conditional and his accounts of the modalities are built on logically relevant temporal properties of propositions (see below sects. 3.2 and 4.2). Moreover, he distinguished between propositions in the present tense like ‘Helen has three husbands’ and ‘These men are marrying’ and propositions in a tense of completion (the aorist), ‘Helen had three husbands’ and ‘These men married’, and observed that it is possible for propositions like the latter two to be true, without there ever having been a time at which a corresponding one of the former type was true (Sextus Empiricus [SE], Against the Physicists 2.97-8).