Abstract: Leibniz held that even if we had no proof of the possibility premise of the ontological argument, a presumption would justify accepting it. He had an extensive theory of presumptions, as a part of practical philosophy, originating in his jurisprudence. He even proposed a formal proof that presumption favors possibility. This chapter examines ways of trying to overcome the difficulty that in the case of a necessary being, where possibility of existence and possibility of nonexistence exclude each other, presumptions of possibility seem to cancel each other out, but concludes that prospects of escape from the problem are not promising.

Keywords: jurisprudence, Leibniz, necessary being, ontological argument, possibility, practical philosophy,