  In all sources the concept of possibility stands out, and so it seems likely that Philo built his set of modal notions on a concept of internal consistency, as given in his account of possibility. Philo's modal concepts are thus defined by resort to another, perhaps more basic, modal concept. As to the kind of consistency Philo had in mind, we learn nothing more. Notwithstanding this, there can be little doubt that Philo's modal concepts satisfy a number of basic requirements which normal systems of modern modal logic tend to satisfy, too. (I assume here that Philo accepted the principle of bivalence, as we have no contrary information.) These requirements are: