As early as in his thesis of Law in 1666, Leibniz aims at founding legal doctrine and practice on a principle providing them with almost scientific certainty and rightness. Two are the obstacles typical of the difficulty of Law oppposing such a project. First, the very contingency of events, therefore of stated cases, leads to a neverending regression in the way causes and motivations justifying facts are analysed. Second, Law history and doctrine create antinomies, because they are the result of judicial acts compiled without order and therefore sometimes incompatible. To overcome this double difficulty and give Law its real value of truth and fairness can only mean discovering a founding principle for the order of facts and the order of reasons. at stake for a philosophy of Law is to prove the existence of such a principle and to state it. But Leibniz doesn't stop at a theoretical level of demonstration: he finds in german recent judicial history an outstanding confirmation of his analysis. This inductions, that Spee draws, indeed, from his own experience during the trials for wichery fully cooroborate Leibniz's logical results: this meeting point makes it therefore possible to give to the image of the wise man incarnating human reason and justice a concrete content.