His work in set theory considered a function as a set of ordered pairs and this made the function notion as proposed by Frege, Charles Peirce and Schröder redundant.
Kanyang trabaho sa set theory itinuturing na isang function bilang isang set ng mga ordered pares na ito at ginawa ang function paniwala bilang ng mga iminungkahing Frege, Charles Peirce at Schröder maulit.