**The 18th and 19th centuries**

In the 18th century there were three major contributors to the development of formal logic: Ploucquet, Lambert, and Euler, although none went far beyond Leibniz and none influenced subsequent developments in the way that Boole and Frege later did. Leibniz’ major goals for logic, such as the development of a “characteristic” language; the parallels among arithmetic, algebra, and syllogistic; and his notion of the truth of a judgment as the concept of the predicate being “included in” the concept of the subject, were carried forward by Christian Wolff but without any significant development of a logic, symbolic or otherwise. The prolific Wolff publicized Leibniz’ general views widely and spawned two minor symbolic formulations of logic; that of J.A. Segner in 1740 and that of Joachim Georg Darjes (1714–91) in 1747. Segner used the notation “B < A” to signify, intensionally in the manner of Leibniz, that the concept of B is included in the concept of A (i.e., “All A’s are B’s”).

**Gottfried Ploucquet**

The work of Gottfried Ploucquet (1716–90) was based on the ideas of Leibniz, although the symbolic calculus Ploucquet developed does not resemble that of Leibniz (see illustration). The basis of Ploucquet’s symbolic logic was the sign “>,” which he unfortunately used to indicate that two concepts are disjoint—i.e., having no basic concepts in common; in its propositional interpretation, it is equivalent to what became known in the 20th century as the “Sheffer stroke” function (also known to Peirce) meaning “neither . . . nor.” The universal negative proposition, “No A’s are B’s,” would become “A > B” (or, convertibly, “B > A”). The equality sign was used to denote conceptual identity, as in Leibniz. Capital letters were used for distributed terms, lowercase ones for undistributed terms. The intersection of concepts was represented by “+”; the multiplication sign (or juxtaposition) stood for the inclusive union of concepts; and a bar over a letter stood for complementation (in the manner of Leibniz). Thus “Ā” represented all non-A’s, while “ā” meant the same as “some non-A.” Rules of inference were the standard algebraic substitution of identicals along with more complicated implicit rules for manipulating the nonidentities using “>.” Ploucquet was interested in graphic representations of logical relations—using lines, for example. He was also one of the first symbolic logicians to have worried extensively about representing quantification —although his own contrast of distributed and undistributed terms is a clumsy and limited device. Not a mathematician, Ploucquet did not pursue the logical interpretation of inverse operations (e.g., division, square root, and so on) and of binomial expansions; the interpretation of these operations was to plague some algebras of logic and sidetrack substantive development—first in the work of Leibniz and the Bernoullis, then in that of Lambert, Boole, and Schröder. Ploucquet published and promoted his views widely (his publications included an essay on Leibniz’ logic); he influenced his contemporary Lambert and had a still greater influence upon Georg Jonathan von Holland and Christian August Semler.

**Johann Heinrich Lambert**

The greatest 18th-century logician was undoubtedly Johann Heinrich Lambert. Lambert was the first to demonstrate the irrationality of π, and, when asked by Frederick the Great in what field he was most capable, is said to have curtly answered “All.” His own highly articulated philosophy was a more thorough and creative reworking of rationalist ideas from Leibniz and Wolff. His symbolic and formal logic, developed especially in his Sechs Versuche einer Zeichenkunst in der Vernunftlehre (1777; “Six Attempts at a Symbolic Method in the Theory of Reason”), was an elegant and notationally efficient calculus, extensively duplicating, apparently unwittingly, sections of Leibniz’ calculus of a century earlier. Like the systems of Leibniz, Ploucquet, and most Germans, it was intensional, using terms to stand for concepts, not individual things. It used an identity sign and the plus sign in the natural algebraic way that one sees in Leibniz and Boole. Five features distinguish it from other systems. First, Lambert was concerned to separate the simpler concepts constituting a more complex concept into the genus and differentia —the broader and narrowing concepts—typical of standard definitions: the symbols for the genus and differentia of a concept were operations on terms, extracting the genus or differentia of a concept. Second, Lambert carefully differentiated among letters for known, undetermined, and genuinely unknown concepts, using different letters from the Latin alphabet; the lack of such distinctions in algebra instruction has probably caused extensive confusion. Third, his disjunction or union operation, “ + ,” was taken in the exclusive sense—excluding the overlap of two concepts, in distinction to Ploucquet’s inclusive operation, for example. Fourth, Lambert accomplished the expression of quantification such as that in “Every A is B” by writing “a = mb” (see illustration)—that is, the known concept a is identical to the concepts in both the known concept b and an indeterminate concept m; this device is similar enough to Boole’s later use of the letter “y” to suggest some possible influence. Finally, Lambert considered briefly the symbolic theorems that would not hold if the concepts were relations, such as “is the father of.” He also introduced a notation for expressing relational notions in terms of single-placed functions: in his system, “i = alpha : : c” indicates that the individual (concept) i is the result of applying a function alpha to the individual concept c. Although it is not known whether Frege had read Lambert, it is possible that Lambert’s analysis influenced Frege’s analysis of quantified relations, which depends on the notion of a function.