〈 Berger | Dillon 〉

i would argue that there's no more fundamental principle. it's one of those things that has remained true from classical mechanics all the way down to quantum field theory

it's a principle because there's no (as we now know) any a priori reason why Nature chooses the path that makes the integral over kinetic *minus* potential stationary

but that is a bit circular. in the end, you choose the lagrangian that gives you the right equation.

you're missing the point .. you choose the *general* form of something called a lagrangian and assert that nature must respect the trajectories that make the action stationary .. you need to make definitions some how, right? defining something is not circular

the lagrangian is just a function of coordinates and its derivatives; it can be anything. for me, it is just another way to encode equations.

but it CANT be anything, physically. it must be the kinetic minus potential or else the thing your making stationary has no correlation with reality

like equations cannot be anything for them to match certain physical phenomena. the kinetic minus potential is just another way to express relations between functions and its derivatives

I agree with you that certain kind of equations satisfy the principle while others don't. but that is, in my view, more related to the fact of unitarity than something else.