Well, sure, but that only pushes the question back one step further: who told you which action to pick? Why pick R and not R^2, for GR, for example? [ Just t

Well, sure, but that only pushes the question back one step further: who told you which action to pick? Why pick R and not R^2, for GR, for example? [ Just to avoid someone spending too much time on an explanation IAAPhysicist; yes I understand effective field theory and the preference for Lagrangians built from relevant/marginal operators. ]