 The leading practitioner of the calculus, indeed the leading mathematician of the eighteenth century, was Leonhard  Euler[22]  (1707–83). Philosophically Euler was a thoroughgoing synechist. Rejecting Leibnizian monadism, he favored the Cartesian doctrine that the universe is filled with a continuous ethereal fluid and upheld the wave theory of light over the corpuscular theory propounded by Newton.