Philo and Diodorus each considered the four modalities possibility, impossibility, necessity and non-necessity. These were conceived of as modal properties or modal values of propositions, not as modal operators. Philo defined them as follows: ‘Possible is that which is capable of being true by the proposition’s own nature … necessary is that which is true, and which, as far as it is in itself, is not capable of being false. Non-necessary is that which as far as it is in itself, is capable of be