 The sixth book of the In calumniatorem, the above-mentioned De natura et arte, is the exegesis of a passage from the second book of the Aristotelian Physics. Written before the other books composing the In calumniatorem, it is the only one in which Bessarion openly names George as his adversary and—as stated above—is the first work in which Bessarion expresses conciliaristic views about Plato and Aristotle. In this sense the De natura et arte represented a sort of “dress rehearsal” for the magnum opus applied to a single exegetical problem, which had nonetheless significant implications for crucial subjects (such as the doctrine of Ideas and divine causality in the world).