Birthday of Frank Ramsey (1903)

Frank Ramsey proved in 1930 that complete disorder is impossible in sufficiently large structures. In any group of 6 people, there must be either 3 mutual friends or 3 mutual strangers. This is R(3,3) = 6. The general question — how large must a group be to guarantee a certain level of order? — defines Ramsey theory, a field still active today. Ramsey died at 26, yet also transformed logic, economics, and probability theory. His work on subjective probability and rational belief influenced how we now think about Bayesian reasoning.