For a long time, I have appreciated the fact that Hamilton's principle of least action can be derived in a straightforward manner via the Feynman path integral (FPI) technique. (This is explained very nicely in the classic textbook: .) Since Hamilton's principle of least action leads to Newtonian mechanics, then we can say that Newtonian mechanics may be derived from quantum mechanics. Recently, I have learned that Einstein's equations of general relativity may be derived from a similar action principle, in a manner analagous to the derivation of Newtonian mechanics from Hamilton's principle. (This is discussed in MTW, section 17.5, box 17.2, section 2.) Here is the difference: in Hamilton's principle, the action is defined as the one-dimensional integral of the lagrangian corresponding to a particle path; but for the derivation of GR, the action is defined iiuc as a 4-dimensional volume integral of a lagrange density. Call the first "action principle #1" (Hamilton's principle), and the second "action principle #2". We have: QM (FPI) ==> action principle #1 ==> Newtonian mechanics as well as action principle #2 ==> GR So here's my question: is it possible to derive "action principle #2" from QM? If so, could we conclude that GR follows necessarily from QM? If not -- is there any other way to derive action principle #2? David  R.P Feynman and A.R. Hibbs. Quantum Mechanics and Path Integrals (McGraw-Hill, Boston, MA, 1965) PS: I should perhaps point out that Hamilton's principle of least action should actually be called the principle of extremal action. Mathematically, the classical action is a stationary point -- not necessarily a minimum.