Logic is concerned with the principles of valid inference; and it is certain that men made inferences and criticized the inferences of others long before the time of Aristotle. This is not enough in itself to justify us in saying that there must have been a beginning of logic before the time of Aristotle; for men may perform various activities correctly (e.g. talk English) without formulating the rules for those activities explicitly. But it is clear from what we find in Plato and Aristotle and other sources that Greek philosophers had begun to discuss the principles of valid inference before Aristotle wrote those works which came to be known as the Organon.
(William Kneale and Martha Kneale, The Development of Logic [Oxford: Clarendon Press, 1962; paperback ed., 1984], 1)
