The notion of validity and the systematic codification of valid forms of deductive argument are central to the discipline of logic. This chapter will reconstruct how, in antiquity, Aristotle and the Stoics constructed two different deductive systems meant to capture and codify an especially important subset of valid inferences, which they called ‘syllogisms’.1 In the process, we will also emphasise some key similarities and differences between the two systems, and between both of them and modern conceptions of validity and formal logic. Because of space limitations we will not include in our presentation other interesting and for the most part subsequent developments in the classification of valid forms of inference.2

In late antiquity interpreters of Plato’s philosophy insisted that the whole of logic was already present in his dialogues. All kinds of syllogisms were used by Socrates and his interlocutors, and it was left to Aristotle and his successors only to name, classify and formalise them.1 This approach remained popular among interpreters until the first half of the twentieth century.2 More recent historians of logic have protested that in order to ‘discover’ or ‘invent’ logic it is not sufficient to reason according to certain valid patterns, or to represent someone acting in this way in a fictional dialogue. But there is a sense in which Plato did play a key role in the birth and development of ancient logic, a role which is often underplayed in histories of logic. In his dialogues Plato identified and explored a number of central philosophical issues to which logical concepts and methods offered powerful responses, if not definitive solutions. In this way, he was an essential catalyst for the birth of logic: if ancient logic was the promised land, Plato was its Moses. He never set foot in it, but enabled others to see the destination. Of course, when setting this agenda, Plato was not operating in a philosophical vacuum; often he was engaging in original ways with problems raised or foreshadowed by some of his predecessors and contemporaries (on the ‘prehistory’ of logic see Chapter 1 – Denyer).

The Greeks invented the philosophical discipline known as ‘logic’, whose core is the study and classification of valid forms of inference. Since its inception Greek logical inquiry was motivated by the need to establish the standards of correctness for philosophical reasoning and argument. Throughout antiquity this inquiry also focused on the identification, diagnosis, and classification of forms of argument that are invalid, unsound, or otherwise problematic. Within these, special attention was devoted to those forms of argument that, despite their deficiency, somehow appear to be valid, and thus can easily induce us in error, or can be exploited ‘sophistically’ to mislead others. To be able to defend oneself, by detecting the fallacies in someone else’s reasoning, was a valuable skill in a context in which philosophical discourse developed in a dialectical setting, and one’s opponents could use, whether consciously or inadvertently, fallacious arguments to (apparently) refute one’s side of the argument and to win the debate. In addition, the study of fallacies was deemed important to avoid errors in one’s own reasoning, which was construed by Plato as a sort of inner, silent dialogue which one entertains with oneself.1