Gottfried Wilhelm Leibniz (1646–1716) was distinguished in many fields, but in none more than in logic. There, however, his worth was not fully appreciated until the twentieth century. He early began to investigate Aristotelian syllogistic and never completely escaped from the syllogistic point of view. In 1666 he wrote a Dissertatio de Arte Combinatoria, a juvenile work that was not free of mistakes, as he later realized, but that showed a new, high sense of organization and a genuine feeling for formal logic, very rare at the time. In one part of this book Leibniz worked out for himself the calculations of Hospinianus (1560) relative to the possible and the valid moods of syllogism. He differed from Hospinianus in making singular propositions equivalent to universal ones, as did Wallis and Euler. He arrived at twenty-four strictly Aristotelian syllogisms, six in each of four figures, which he arranged in a neat tableau...