If I recall correctly he talked further about this in one of his autobiographies (Surely You’re Joking, Mr Feynman possibly?).
It was about the repeatability of counting to a minute, and he experimented on the difference between seeing a timer tick in your minds eye and hearing a count increment in your head.
He had been experimenting with what he could do while counting, and found he couldn't count while talking, but could count while reading. John Tukey didn't believe him, and thought it should be the other way around. It turned out Tukey and Feynman counted completely differently, Tukey by visualising and Feynman using internal speech, and so Tukey could count while talking, but not while reading. So, not the same person doing both methods.
It was about the repeatability of counting to a minute, and he experimented on the difference between seeing a timer tick in your minds eye and hearing a count increment in your head.