Megarian dialecticians – Diodorus Cronus and Philo – had worked in this field and the pupils of Aristotle – Theophrastus and Eudemus – had investigated hypothetical syllogisms, it was Chrysippus who developed these principles into a coherent system of propositional logic. Propositions Chrysippus defined a proposition as "that which is capable of being denied or affirmed as it is in itself" and gave examples of propositions such as "it is day" and "Dion is walking." He distinguished between simple and non-simple propositions, which in modern terminology are known as atomic and molecular propositions. A simple proposition is an elementary statement such as "it is day." Simple propositions are linked together to form non-simple propositions by the use of logical connectives. Chrysippus enumerated five kinds of molecular propositions according to the connective used: Thus several types of molecular propositions, familiar to modern logic, were listed by Chrysippus, including the conjunction, the disjunction, and the conditional, and Chrysippus studied their criteria of truth closely. Conditional propositions The first logicians to debate conditional statements were Diodorus Cronus and his pupil Philo. Writing five-hundred years later, Sextus Empiricus refers to a debate between Diodorus and Philo. Philo regarded all conditionals as true except those which with a correct antecedent had an incorrect consequent, and this meant a proposition such as "if it is day, then I am talking," is true unless it is day and I fall silent. But Diodorus argued that a true conditional is one in which the antecedent clause could never lead to an untrue conclusionthus, because the proposition "if it is day, then I am talking" can be false, it is invalid. However, paradoxical propositions were still possible such as "if atomic elements of things do not exist, atomic elements exists." Chrysippus adopted a much stricter view regarding conditional propositions, which made such paradoxes impossible: to him, a conditional is true if denial of the consequent is logically incompatible with the antecedent. This corresponds to the modern-day strict conditional. Syllogistic Chrysippus developed a syllogistic or system of deduction in which he made use of five types of basic arguments or argument forms called indemonstrable syllogisms, which played the role of axioms, and four inference rules, called themata by means of which complex syllogisms could be reduced to these axioms. The forms of the five indemonstrables were: Of the four inference rules, only two survived. One, the so-called first thema, was