  Peirce introduced the material-conditional operator into logic, developed the Sheffer stroke and dagger operators 40 years before Sheffer, and developed a full logical system based only on the stroke function. As Garret Birkhoff notes in his Lattice Theory it was in fact Peirce who invented the concept of a lattice (around 1883).  (Quite possibly, it is Peirce’s lattice theory that holds the key to his technical theory of infinitesimals and the continuum.)