  Pre-Aristotelian evidence for reflection on argument forms and valid inference are harder to come by. Both Zeno of Elea (born c. 490 BCE) and Socrates (470–399) were famous for the ways in which they refuted an opponent’s view. Their methods display similarities with reductio ad absurdum, but neither of them seems to have theorized about their logical procedures. Zeno produced arguments (logoi) that manifest variations of the pattern ‘this (i.e. the opponent’s view) only if that. But that is impossible. So this is impossible’. Socratic refutation was an exchange of questions and answers in which the opponents would be led, on the basis of their answers, to a conclusion incompatible with their original claim. Plato institutionalized such disputations into structured, rule-governed verbal contests that became known as dialectical argument. The development of a basic logical vocabulary for such contests indicates some reflection upon the patterns of argumentation.