  Modal logic is the second topic where we have evidence about the positions of Philo and Diodorus and their influence on the Stoics (Kneale and Kneale 1962, 117-28; Bobzien 1993). Although the modalities were generally discussed under the heading of ‘On things possible’, the Hellenistic modal systems were each built on a set of four modalities: possibility, impossibility, necessity and non-necessity. The matter of dispute was which system was the right one; that is, which one adequately described the modalities inherent in the world. In connection with this, an extra-logical concern provided additional fuel to the debate: the belief that if propositions stating future events that will not happen all turn out to be impossible, the freedom and choices of individuals would be curtailed. This is a variation on the problem of logical determinism which is known from Aristotle's On Interpretation 9. Several of the sophisms devised and discussed by the Dialectical school touch upon this issue — the Mower Argument, the Lazy Argument (Bobzien 1998, 78-81, 180-233), and the Master Argument.