I was looking at the wikipedia article on CPT and it starts with "Charge, Parity, and Time Reversal Symmetry is a fundamental symmetry of physical laws under the simultaneous transformations of charge conjugation (C), parity transformation (P), and time reversal (T)." What does it mean that CPT is a symmetry of the "physical laws?" Is it obvious that a QFT has CPT symmetry from its Lagrangian? For example, if I had a Lagrangian for some QFT, and replaced the particle fields with anitparticle fields and vise versa, does C symmetry imply that the two conjugate Lagrangians will yield the same equations of motion? If so, how could a parity or time inversion operation be applied to a Lagrangian in the same way? What is the precise meaning of "physical laws?"