It must be pointed out that there is a minimum of arbitrariness in the choice of these equations. For besides

there is no tensor of second rank which is formed from the

and its derivatives, contains no derivations higher than second, and is linear in these derivatives.*
These equations, which proceed, by the method of pure mathematics, from the requirement of the general theory of relativity, give us, in combination with the equations of motion ([equation number removed]), to a first approximation Newton's law of attraction, and to a second approximation the explanation of the motion of the perihelion of the planet Mercury discovered by Leverrier (as it remains after corrections for perturbation have been made). These facts must, in my opinion, be taken as a convincing proof of the correctness of the theory."