Thanks to philosopher Cheyney Ryan (Oxford) for sharing this anecdote from Ali's biography:

When Ali encountered a distinguished, ninety-something English fellow prior to his fight with Henry Cooper, he asked him his opinion on the outcome.

“Our ‘Enry’s capable but I think you’ll win.”

Ali responded, “You’re not as dumb as you look.”

He had no idea at the time he was addressing the eminent philosopher and pacifist Bertrand Russell and later wrote to him to apologise. Fortunately, the author of Principia Mathematica took no offence at Ali’s backhanded compliment and the pair exchanged correspondence until Russell’s death in 1970, with Russell writing Ali letters of encouragement concerning his stand on the Vietnam war.

(Russell corresponded with Ali offering material from his Vietnam War Crimes Tribunal for Ali's legal defense for refusing conscription.) 

As a kid in Manhattan, I can still recall huge posters advertising "Cassius Clay."   I'm pretty sure they were leftover from before he changed his name.