 The examples dealt with so far are examples of simple, complete sayables or propositions. The Stoics also developed an account of non-simple propositions. This interest in non-simple propositions and their logical relations was shared with philosophers in the Megarian or Dialectical school. It set the philosophers of the Hellenistic period on the pathway to surpass Aristotle’s progress in logic. His logic was ‘a logic of terms’. To put the matter very briefly and far too crudely, Aristotle had developed an account of a limited range of kinds statements (e.g. All A are B, or Some A are B, or No A are B). His theory of the syllogism sought to systematically investigate all the ways of combining pairs of such statements and to identify the combinations where the first two (the premises) entail a third statement (the conclusion) of same sort purely as a result of the form of the premises rather than their content. Focused on the connections between predicate and subject terms in such statements, it had little to say about complex statements that had complete statements as parts. The Stoics, by contrast, made progress in what we now call propositional logic. They developed accounts of propositional negation (‘it is not the case that p’), conjunction (‘p and q’), disjunction (‘p or q’) and entered the on-going debate over the correct understanding of conditionals (‘if p, then q’). Their accounts of the connectives joining simple propositions into complex ones also led them into questions about modal concepts (possibility, impossibility, necessity and contingency). One of the accounts they offer of the validity of arguments is that an argument is valid if, through the use of certain ground rules (themata), it is possible to reduce it to one of the five indemonstrable forms (Diog. Laert., 36A). These five indemonstrables are argument forms that should be familiar to anyone who has taken an introductory logic class: