[anon_123g987]

Thanks for the correction! I also think that both "in the head" and "on paper" work is necessary, of course, and I also think that Feynman did both, like everyone else. But we know that he often presented himself in the way the Gell-Mann quote describes him; he purposefully cultivated his image as a genius who can solve complicated problems in his head in seconds (if I recall correctly, he "pre-solved" problems, waited until the problems came up in a conversation and came up with the memorized solution after five seconds of "thinking").