To those who think of Leibniz as a dogmatic philosopher, his close association with a sceptic such as Simon Foucher may come as a surprise. Thus, in a characteristically ground-breaking article, Richard Popkin remarks that it was “strange but nevertheless a fact that Leibniz was a warm and close friend of the three leading French sceptics of his time, the abbé Simon Foucher, Bishop Pierre-Daniel Huet and Pierre Bayle” (1966: 228). Popkin's investigation into how such an accord could be maintained between such apparently contrary types of philosopher led him to acknowledge that Leibniz, after all, was not as far from the sceptics as is commonly supposed. And on this point, on which I will have more to say, I agree with him. But, at least as far as Foucher goes, another part of the explanation is that someone may call himself a sceptic because he is committed to the search after truth and not, as Popkin supposes, because of an inclination to doubt everything. In fact, or so I will argue, Foucher and Leibniz agreed about a good deal and, for a while, thought they agreed even more, about philosophical matters. There was, as the title of my chapter claims, an alliance between them. And this alliance had, or appeared to each of them to have, substantial philosophical bases.