Gottlob Frege (1848-1925)
Frege, a German mathematician and philosopher, founded the discipline of logic in its modern form.
In publishing his best known book Begriffsschrift, Frege invented the very first predicate calculus, formalized the system of proofs, and formed the basis of what he would later devote his life to: an unsuccessful attempt at showing that mathematics was reducible to logic (Bertrand Russell showed that Russell's paradox made one of his axioms contradictory).
Despite this, Frege's work was a significant advance in the field of logic, an area that had been basically stagnant since the medieval revival of Aristotelian logic . He is thus called by many as the "father of modern logic". Frege had also made a significant contribution to the philosophy of language by suggesting that an expression had both a sense and a denotation -- an idea that spawned significant research for a century. Russel, Carnap, and Wittgenstein were some of the many thinkers strongly influenced by Frege.
Frege was a conservative German, which at that time meant that he was quite distrustful of foreigners and was anti-semitic.
Frege wrote little after 1903, apparently depressed at the poor reception of his ideas.
References:
http://www.utm.edu/research/iep/f/frege.htm
http://plato.stanford.edu/entries/frege/
http://mally.stanford.edu/frege.html
Frege, Gottlob, biography.com