 algebra |  Gödel, Kurt |  Leśniewski, Stanisław |  liar paradox |  logic: classical |  logic: second-order and higher-order |  logical consequence |  logical constants |  logical truth |  Lvov-Warsaw School |  model theory |  reference |  Russell, Bertrand |  set theory |  Tarski, Alfred: truth definitions |  truth |  type theory |  Vienna Circle |  Whitehead, Alfred North 