 It is no surprise that Leibniz chose not to publish the Discours. It is also no surprise that the complete concept theory of substance does not make an appearance in the Système nouveau. This does not imply that Leibniz abandoned the complete concept theory; it might just be that he came to realize that it was not the most effective way of introducing readers to his philosophical system. Leibniz utilizes the exoteric strategy of selective omission, choosing to avoid a detailed discussion of necessity and contingency. Instead he introduces his theory of substance by appealing to the less controversial characteristics of unity and activity, and by trying to show how his theory could resolve the notorious problem of mind-body interaction. Letters to some of Leibniz’s correspondents confirm that he was “testing the waters” in the Système nouveau, and that he would reveal more of his system to the public if it was well received. He writes to Simon Foucher, for example: