David Hilbert
One of the greatest mathematicians in the 20th century, and a formalist philosopher of mathematics. Aside from many creative works in mathematics, he refined the axiomatic method (Grundlagen der Geometrie, 1899, and later works on the foundations of mathematics), proposed the Finitism (mathematics within strict finite means), and advocated Hilbert's Program which seeks to prove consistency of an axiomatized mathematics by means of Finitism. Although this program is now generally considered to have failed, it inspired many logicians such as Bernays, Goedel, Gentzen, or Herbrand.