Serge Lang started out as a graduate student of philosophy at Princeton, but he switched to math, because he had "finished it", it being philosophy. 

Here's the relevant part from his biography [here][1]

"After returning to the United States, Lang went to Princeton University with the intention of studying for a doctorate in philosophy. After a year in the philosophy department, he changed to mathematics and Emil Artin became his thesis advisor."


  [1]: http://www.gap-system.org/~history/Biographies/Lang.html