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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.

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# Why does Lagrangian Density only depend on field variables and their first derivative

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