The history of logic in the West has three great periods. The first was in Ancient Greece, when logic was founded by Aristotle, the Megarians, and the Stoics. The second was in the new universities of Medieval Europe, such as Oxford and Paris, where Ockham, Scotus, and Buridan flourished. The third starts in the late nineteenth century, with the rise of mathematical logic, and shows no signs yet of ending. A logica docens may be revised rationally, and this happens by the standard mechanism of rational theory choice. A logica utens may be changed by bringing it into line with the logica docens; and if the docens is chosen rationally, so is the utens. The answer to the question of whether or not the logica ens may change depends on one's best answer to the question of what validity is.