Yeah, that's what I got too. On a more substantive note, it's actually absolutely amazing what Feynmann managed to pull off using 'just' calculus. Especially

Yeah, that's what I got too. On a more substantive note, it's actually absolutely amazing what Feynmann managed to pull off using 'just' calculus. Especially if you read the Feynmann lectures, when it comes to integrals and calculus, he had unmatched mastery and understanding. I've never really stopped to think about that before now.