Carneades

Carneades led the Academy in Athens during its most thoroughly skeptical phase, continuing a tradition begun a century earlier when the school Plato founded turned against Plato's own confident metaphysics and made systematic doubt its method instead. He argued that certainty is unreachable, but unlike more radical skeptics he did not conclude that all beliefs are therefore equally worthless — he developed a theory of degrees of plausibility, arguing that some impressions are more probable than others and that a reasonable person acts on the most convincing available impression while withholding final assent, a middle path between dogmatism and paralysis that later influenced Cicero's own philosophical outlook directly. His most famous performance came in 155 BCE, when Athens sent him to Rome as part of a diplomatic delegation. He lectured on justice on consecutive days: first a polished defense of justice as a natural virtue, then, to the horror of the assembled Romans, an equally polished demolition of the same position, arguing that justice was merely convention and that empires, Rome's included, were built on organized theft. Cato the Elder, alarmed that Roman youths were being taught to argue any side of any question with equal skill, pushed to have the philosophical delegation sent home as quickly as possible. Carneades wrote nothing down, delivering his arguments entirely through oral disputation and lecture, so everything known about his positions survives only through students and later writers, chiefly Cicero and Sextus Empiricus, who preserved and sometimes argued with a mind so allergic to fixed conclusions that he reportedly refused to let even his own school's doctrines calcify into dogma.