Like many controversies in science, the one between Freud and Jung is overloaded with ad hominem arguments despite the incompatibility of such arguments with the pretensions of both sides to attain scienfific ad rem validity. Unlike natural scientists, Freud and Jung regarded their own ad hominem arguments as relevant to general and impersonal truths. They practically legitimized such a use claiming to have a clinical basis for the rejection of the opponent's objections by a de-validating analysis of the opponent's personality as a whole. The argument of this paper is that the de-validating strategy was neither an inevitable psychological outcome of the intricate interpersonal relationships in analytic situations nor the logical consequence of any clinical or scientific psycho-analytical discovery. It followed from the epistemological invalid pretension to have a general theory of mind which could explain by mental analysis the existence of "unreasonable opinions," and the application of the same principles to the opinion that the theory itself is unreasonable. Such a pretension, apparently specific to mystical traditions in theology and metaphysics, was deeply rooted in the modern epistemological tradition. The paper examines the impact of the differenct branches of that tradition on Freud and Jung's respective ideologies, theories, and practice, including the ad hominem malpractice.