Justice Stevens: "that was the day I decided to resign... I learned giving that talk that I had a speech problem."

"That talk" = the announcement of his dissenting opinion in Citizens United, which you can listen to here.

From an interview published today in The Atlantic.

Stevens said he retired because, while he still loved the job of judging, he had no desire to linger beyond his physical prime. He had witnessed the final years on the bench of [William O.] Douglas, Thurgood Marshall and others who should have retired earlier for health reasons. A few years ago, he secretly asked Associate Justice David Souter to tell him when it was time for him to go. But Souter left first, in 2009.

"When he retired, I knew I didn't have any safety valve anymore."

The suggestion, as I read it, is that Stevens had to judge himself strictly because he didn't have Souter to reassure him that the time to go had not yet arrived. (How can you tell if you've lost your mental powers?)

Why Souter was a unique confidante, the interviewer did not ask.