This paper is a study on the Leibnizian conception of the combination of human thoughts as a proof involved with the logic of Leibniz. There are many aspects in Leibniz's thought about logic, which are reflected in his various logical ideas and projects. This paper attempts to illuminate that these ideas and projects have important philosophical implications, that they consist of his logic as a scientia universalis, and that his logic is an ars judicandi based on the combination of thoughts and the manipulation of signs. First of all, Leibniz introduced various mathematical methods to establish his logical system. And he needed to perform preliminary logical projects such as the alphabet of human thoughts and the new encyclopedia. The former is the list of signs for simple concepts founded by analyzing complex concepts, and the latter yields to simple concepts and definitions involved in all the sciences. On the basis of these, he attempted to construct his logical system. It enables us to demonstrate all the scientific propositions and to use a characteristica universalis and a calculus ratiocinator. Although he failed to build up a distinct logical system, he endeavored to devise a logical notation for the first time, and his conception of logic inspired Boole and Frege to found modern logics.