In 1915 David Hilbert applied variational principles to derive the gravitational field equations of general relativity in agreement with Albert Einstein 's derivation. (Einstein and Hilbert discussed Einstein's work on general relativity in person and letters throughout 1915.) Hilbert's approach required accepting the variational principle as "axiomatic", a broadly accepted requirement today but questionable to the physicists of 1915. Hilbert's variations were based on what became known as the Einstein–Hilbert action, given by