Hi all, As a blind follower of QFT from the sidelines (the joys of the woefully inadequate teaching of theory to exp. particle physics students...), I have just realised that I've never actually gone further than deriving the Dirac equation, and then just used the Dirac Lagrangian density as the basis for learning about gauge theories. What is not clear to me is how to move from the Dirac equation to the Dirac Lagrangian density ([tex]\bar\psi\left(i\gamma^\mu\delta_\mu -m\right)\psi[/tex]). I've been playing around with a few ideas but am getting absolutely nowhere. There is a glib explanation in P&S, but it struck me as somewhat circular and I wasn't convinced at all. Srednicki doesn't cover it either, as far as I can see (I have the former in front of me, the latter on my desk at work...). Any help welcome! Cheers.