Jeff mentioned he thought a thread on this might be good, and I decided to start it here so the board wouldn't be entirely blue sky. I'll start with a question raised by Patrick van Esch on s.p.r. In the Peskin and Schroeder textbook, they develop the expansion of the wave φ in the free theory into normal modes with the ladder operators a and a+. So far so good. Then when they come to discuss interacting field theory, they exploit that same expansion, ladder operators and all, for their development. But they haven't redefined their expansion technology in terms of the new "interacting" φ, so where do they get off doing this? All comments welcome but references to Haag's theorem should be tied to the issue at hand.