Boxing legend and activist Muhammad Ali relied on a four-letter word as his secret weapon one that documentarian Ken Burns said is so uncomfortable, we dont know how to talk about it.

That word: Love.

Burns, whose upcoming documentary on the late boxing champ is set to hit PBS later this month, spoke to CNN s John Avlon on Sunday about how Alis legacy transcended what he accomplished in sports. He also talked about how the boxer, nicknamed The Greatest, intersected with all of the most important issues of the second half of the 20th century, including race, politics, war and faith.

This is a man who grew, who was an avatar a prophet of love, Burns said. He couldve been anything. And he knew from a very early age that he had a mission, and that was much bigger than boxing. When he died, he died the most popular person on the planet which, by the way, is not a bad thing for any of us to try to head toward.

See more of their conversation above.

Muhammad Ali premieres on PBS on Sept. 19.