Dear Vaughan, I agree with your definition: "A proof is sufficient evidence for the truth of a proposition," The article http://en.wikipedia.org/wiki/Proof does not discuss the idea (of Paul Lorenzen) that a mathematical proof is essentially a winning strategy in a formal game. I first learned the idea from Andreas Blass who introduced the game semantic of linear logic, http://arxiv.org/abs/math/9310211 A proof can be viewed as an argumentation to convince others of the validity of a statement. In mathematics, the argumentation must be solid enough to resist any conter-argumentation by an ideal opponent. It can be compared to a winning strategy in a game with two players, one defending a statement and the other attacking it. Lorenzen associates to every mathematical statement S a formal game with two players G(S), the defender and the attacker. The defender has a winning strategy iff the statement has a formal proof. The rules of the games for a proof in intuitinistic logic differ from the rules for a proof in classical logic. In other words, the rules of the games are determining the logic and vice versa. I believe that game semantic is putting some light on the origin of logic. I guess that logic was discovered by peoples debating in a democratic manner. All communities need to choose between different courses of actions. There are many answers to the question: how should this choice made? One was given by Plato who favored a government by the "philosopher king" who "loves the sight of truth": http://en.wikipedia.org/wiki/Plato#The_State Plato does not like Athenian democracy because it is imperfect. He observes that its political debates are manipulated by sophists. I agree with Plato that democracy is imperfect. But it should be improved, not condemned. Logic is anti-authoritarian since it wishes to convince, not to coerce. Best, André [For admin and other information see: http://www.mta.ca/~cat-dist/ ]