Modern Logic began in 1879, the year in which Gottlob Frege (1848–1925) published his Begriffsschrift. In less than ninety pages this booklet presented a number of discoveries that changed the face of logic. The central achievement of the work is the theory of quantification; but this could not be obtained till the traditional decomposition of the proposition into subject and predicate had been replaced by its analysis into function and argument(s). A preliminary accomplishment was the propositional calculus, with a truth-functional d of the connectives, including the conditional. Of cardinal importance was the realization that, if circularity is to be avoided, logical derivations are to be formal, that is, have to proceed according to rules that are devoid of any intuitive logical force but simply refer to the typographical form of the expressions; thus the notion of formal system made its appearance. The rules of quantification theory, as we know them today, were then introduced. The last part of the book belongs to the foundations of mathematics, rather than to logic, and presents a logical definition of the notion of mathematical sequence. Frege’s contribution marks one of the sharpest breaks that ever occurred in the development of a science. The interest in logic, so alive in the Middle Ages, had died out at the end of the fifteenth century, and the great thinkers of the next three centuries were not attracted by logical studies. In 1787 Kant wrote that logic ‘is to all appearance complete and perfect’. One great philosopher, Leibniz (1646–1716), showed a profound and constant interest in logic. But Leibniz’s investigations are more a program than a realization. He never
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