A development in Germany originally completely distinct from logic but later to merge with it was Georg Cantor’s development of set theory. In work originating from discussions on the foundations of the infinitesimal and derivative calculus by Baron Augustin-Louis Cauchy and Karl Weierstrass, Cantor and Richard Dedekind developed methods of dealing with the large, and in fact infinite, sets of the integers and points on the real number line. Although the Booleans had used the notion of a class, they rarely developed tools for dealing with infinite classes, and no one systematically considered the possibility of classes whose elements were themselves classes, which is a crucial feature of Cantorian set theory. The conception of “real” or “closed” infinities of things, as opposed to infinite possibilities, was a medieval problem that had also troubled 19th-century German mathematicians, especially the great Carl Friedrich Gauss. The Bohemian mathematician and priest Bernhard Bolzano emphasized the difficulties posed by infinities in his Paradoxien des Unendlichen (1851; “Paradoxes of the Infinite”); in 1837 he had written an anti-Kantian and pro-Leibnizian nonsymbolic logic that was later widely studied. First Dedekind, then Cantor used Bolzano’s tool of measuring sets by one-to-one mappings; using this technique, Dedekind gave in Was sind und was sollen die Zahlen? (1888; “What Are and Should Be the Numbers?”) a precise definition of an infinite set. A set is infinite if and only if the whole set can be put into one-to-one correspondence with a proper part of the set. (De Morgan and Peirce had earlier given quite different but technically correct characterizations of infinite domains; these were not especially useful in set theory and went unnoticed in the German mathematical world.)

Although Cantor developed the basic outlines of a set theory, especially in his treatment of infinite sets and the real number line, he did not worry about rigorous foundations for such a theory—thus, for example, he did not give axioms of set theory—nor about the precise conditions governing the concept of a set and the formation of sets. Although there are some hints in Cantor’s writing of an awareness of problems in this area (such as hints of what later came to be known as the class/set distinction), these difficulties were forcefully posed by the paradoxes of Russell and the Italian mathematician Cesare Burali-Forti and were first overcome in what has come to be known as Zermelo-Fraenkel set theory.

Other 19th-century logicians

French logic was ably, though not originally, represented in this period by Louis Liard and Louis Couturat. Couturat’s L’Algèbre de la logique (1905; The Algebra of Logic) and De l’Infini mathematique (1896; “On Mathematical Infinity”) were important summaries of German and English research on symbolic logic, while his book on Leibniz’ logic (1901) and an edition of Leibniz’ previously unpublished writings on logic (1903) were very important events in the study of the history of logic. In Russia V.V. Bobyin (1886) and Platon Sergeevich Poretsky (1884) initiated a school of algebraic logic. In the United Kingdom a vast amount of work on formal and symbolic logic was published in the best philosophical journals from 1870 until 1910. This includes work by William Stanley Jevons, whose intensional logic is unusual in the English-language tradition; John Venn, who was notable for his (extensional) diagrams of class relationships (see illustration) but who retained Boole’s noninclusive class union operator; Hugh MacColl; Alexander Bain; Sophie Bryant; Emily Elizabeth Constance Jones; Arthur Thomas Shearman; Lewis Carroll (Charles Lutwidge Dodgson); and Whitehead, whose A Treatise on Universal Algebra (1898) was the last major English logical work in the algebraic tradition. Little of this work influenced Russell’s conception, which was soon to sweep through English-language logic; Russell was more influenced by Frege, Peano, and Schröder. The older nonsymbolic syllogistic tradition was represented in major English universities well into the 20th century by John Cook Wilson, William Ernest Johnson, Lizzie Susan Stebbing, and Horace William Brindley Joseph and in the United States by Ralph Eaton, James Edwin Creighton, Charles West Churchman, and Daniel Sommer Robinson.

The Italian mathematician Giuseppe Peano’s contributions represent a more extensive impetus to the new, nonalgebraic logic. He had a direct influence on the notation of later symbolic logic that exceeded that of Frege and Peirce. His early works (such as the logical section of the Calcolo geometrico secondo l’Ausdehnungslehre di H. Grassman [1888; “Calculus of Geometry According to the Theory of Extension of H. Grassmann”]) were squarely in the algebraic tradition of Boole, Grassmann, Peirce, and Schröder. Writing in the 1890s in his own journal, Revista di mathematica, with a growing appreciation of the use of quantifiers in the first and third volumes of Schröder’s Vorlesungen, Peano evolved a notation for quantifiers. This notation, along with Peano’s use of the Greek letter epsilon, ε, to denote membership in a set, was adopted by Russell and Whitehead and used in later logic and set theory. Although Peano himself was not interested in the logicist program of Frege and Russell, his five postulates governing the structure of the natural numbers (now known as the Peano Postulates), with similar ideas in the work of Peirce and Dedekind, came to be regarded as the crucial link between logic and mathematics. It was widely thought that all mathematics could be derived from the theory for the natural numbers; if the Peano postulates could be derived from logic, or from logic including set theory, its feasibility would have been demonstrated. Simultaneously with his work in logic, Peano wrote many articles on universal languages and on the features of an ideal notation in mathematics and logic—all explicitly inspired by Leibniz.

Logic in the 19th century culminated grandly with the First International Congress of Philosophy and the Second International Congress of Mathematics held consecutively in Paris in August 1900. The overlap between the two congresses was extensive and fortunate for the future of logic and philosophy. Peano, Alessandro Padoa, Burali-Forti, Schröder, Cantor, Dedekind, Frege, Felix Klein, Ladd Franklin (Peirce’s student), Coutourat, and Henri Poincaré were on the organizing committee of the Philosophical Congress; for the subsequent development of logic, Bertrand Russell was perhaps its most important attendee. The influence of algebraic logic was already ebbing, and the importance of nonalgebraic symbolic logics, of axiomatizations, and of logic (and set theory) as a foundation for mathematics were ascendant. Until the congresses of 1900 and the work of Russell and Hilbert, mathematical logic lacked full academic legitimacy. None of the 19th-century logicians had achieved major positions at first-rank universities: Peirce never obtained a permanent university position, Dedekind was a high-school teacher, and Frege and Cantor remained at provincial universities. The mathematicians stayed for the Congress of Mathematics, and it was here that David Hilbert gave his presentation of the 23 most significant unsolved problems of mathematics—several of which were foundational issues in mathematics and logic that were to dominate logical research during the first half of the 20th century.