Gottlieb Frege (1848-1925)

Frege's work with Mathematics was foundational to modern logic and he introduced such working concepts as the conditional and the variable. He also worked on the status of arithmetic's and, although some his proofs were later proved incomplete by Russell, was largely responsible for the establishment of the basic concepts of math on a logical basis.

He also wrote extensively on language in relation to logic, examining ideas like sense, reference, (which Deleuze draws extensively on) concept, identity, assertion/negation. His writings established such ideas as a legitimate area for philosophical investigation, and must be seen as the beginning of the twentieth centuries contest with language and meaning.

1879 `Begriffsschrift' (`Concept writing')
1884 `Grundlagen ser Arithmetik' (`The Foundations of Arithmetic')