The focus of Curry's work were attempts to show that combinatory logic could provide a foundation for mathematics. Towards the end of 1933, he learned of the Kleene–Rosser paradox from correspondence with John Rosser. The paradox, developed by Rosser and Stephen Kleene had proved the inconsistency of a number of related formal systems including one proposed by Alonzo Church (a system which had the lambda calculus as a consistent subsystem) and Curry's own system.[2] However, unlike Church, Kleene, and Rosser, Curry did not give up on the foundational approach, saying that he did not want to "run away from paradoxes."[3]