One of the celebrated moments of the Reformation age occurred in Leipzig in the summer of 1519. The young Wittenberg theologian Luther, with his colleague Carlstadt, engaged in debate with the even younger scholar from Ingolstadt, John Eck. The present article is an assessment of a well-known conclusion reached about that debate. I mean Luther's judgment that Eck “imitated Proteus” and contradicted himself, rejecting against one debater a position he had affirmed against the other. The charge, which Luther published, did nothing to enhance Eck's reputation at a time when he had already cultivated an assortment of enemies on other grounds.