I was flipping through "Surely you're joking, Mr. Feynman" and I came across something he said that I'm curious about. I'll go ahead and quote it: "The book showed how to differentiate parameters under the integral sign - it's a certain operation. It turns out that's not taught very much in the universities; they don't emphasize it. But I caught on how to use that method, and I used that one damn too again and again." My question is, what method is he referring to? I'm only in vector cal, so I don't know if it's a method that is associated with higher-level mathematics, or if it's just something that I've "learned" and forgotten or what. So if anyone could enlighten me as to this method, how it's done, when it's used, etc., I'd be much appreciative. Thanks.