 Brouwer, Luitzen Egbertus Jan |  Gödel, Kurt |  logic, history of: intuitionistic logic |  logic: classical |  logic: modal |  logic: provability |  logicism and neologicism |  mathematics, philosophy of |  mathematics, philosophy of: formalism |  mathematics, philosophy of: intuitionism |  mathematics: constructive |  Platonism: in the philosophy of mathematics |  proof theory: development of |  set theory: constructive and intuitionistic ZF 