Here I have just plagiarized from Wikipedia.

Andrew Mattei Gleason (November 4, 1921 – October 17, 2008) was an American mathematician who as a young World War II naval officer broke German and Japanese military codes, then over the succeeding sixty years made fundamental contributions to widely varied areas of mathematics, including the solution of Hilbert’s fifth problem, and was a leader in reform and innovation in math­e­mat­ics teaching at all levels.[3][4] His entire academic career was at Harvard, from which he retired in 1992 as the Hollis Professor of Mathematics and Natural Philosophy. Gleason’s theorem and the Greenwood–Gleason graph are named for him.

Gleason’s numerous academic and scholarly leadership posts included chairmanship of the Harvard Mathematics Department[5] and Harvard Society of Fellows, and presidency of the American Mathematical Society. He continued to advise the United States government on cryptographic security, and the Commonwealth of Massachusetts on math­e­mat­ics education for children, almost until the end of his life. The Notices of the American Mathematical Society called him “one of the quiet giants of twentieth-century mathematics, the consummate professor dedicated to scholarship, teaching, and service in equal measure.”[6]

He was fond of saying that math­e­mat­ic­al proofs “really aren’t there to convince you that something is true—they’re there to show you why it is true.”[7]