In his Medicina mentis (1686/87; 2nd ed. 1695), Ehrenfried Walther von Tschirnhaus claims to have found an ‘intermediary path’ between a priori and a posteriori methods in natural philosophy—a three-step method of introspection, deduction, and experimentation. In this paper, I offer a systematic reconstruction of this method, showing how it was developed in the context of Tschirnhaus’s complex exchanges with Spinoza and Leibniz during the mid-1670s and early 1680s concerning an originally Cartesian problem, namely what I call ‘the problem of causal equivocity’, according to which a single effect can be explained a priori by a multitude of possible causes. Contrary to both Leibniz and Spinoza, Tschirnhaus’s three-step method proposed a distinctly non-metaphysical solution to this problem, in conformity with his ambition to develop an epistemological foundation for the conduct of experimental natural science.
Mogens Laerke (Thu,) studied this question.