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 P: 611 Here and there I come across the following formula for the Lagrangian density of a real scalar field, but not a deriviation. $$\mathcal{L} = \frac {1}{2} [ \dot \phi ^2 - ( \nabla \phi ) ^2 - (m \phi )^2 ]$$ Could someone show me where this comes from? The m squared term in particular is a mystery.