Bertrand Russell

In more than 50 books penned over 74 years, Bertrand Russell set the terms of the debate in logic and philosophy in the first part of the century--most notably with Principia Mathematica (1910-13), written with philosopher Alfred North Whitehead.

He also married four times, lost three elections to Parliament, founded a school and led the movement for nuclear disarmament. He was twice jailed and dismissed from three jobs for his pacifism and unconventional views on sex. He won the Nobel Prize for Literature in 1950 and died two decades later at 97, a humane rationalist to the last.

--By Unmesh Kher