Hi, guys, Why do we assume Lagrangian Density only depend on field variables and their first derivative? Currently, I am reading Ashok Das's Lectures on Quantum Field Theory. He says (when he is talking about Klein-Gordon Field Theory): "In general, of course, a Lagrangian density can depend on higher order derivatives. However, for equations which are at most second order in the derivatives, the Lagrangian density can depend at the most on the first order derivatives of the field variables. These are the kinds of equations we will be interested in and correspondingly we will assume this dependence of the Lagrangian density on the field variables through out." I am a bit confused about what he says.