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