  Leibniz believes his distinctions also serve to show the difference between true and false ideas. “An idea is true,” he writes, “when its notion is possible and false when it includes a contradiction” (A VI iv 589/AG 26). Now, possibility can be established a priori and a posteriori. On the one hand, we can know a priori that something is possible if we can resolve it into its component notions which are themselves possible and if we know that there is no incompatibility among those component notions. On the other hand, we know a posteriori that something is possible merely through experience, for the actual existence of a thing is proof of its possibility.