The following is a question regarding the derivation of Einstein's field equations. Background In deriving his equations, it is my understanding that Einstein equated the Einstein Tensor Gμv and the Cosmological Constant*Metric Tensor with the Stress Energy Momentum Tensor Tμv term simply because the covariant derivative of all three terms equals zero. Rμv - (1/2)*gμv*R + [itex]\Lambda[/itex]*gμv = (8*pi*G)/(c4)*Tμv Question Is this basis for equivalence (that terms are equivalent if their covariant derivatives are equal) standard practice in mathematics, or did Einstein take a leap of faith? Thank you very much for your time! Please let me know if I can clarify my question.