"Under Grossman's guidance, Einstein studied and learned tensor analysis. The two men actually published several papers together, but the work at this stage was still a kind of groping in the dark—a mind-tormenting search for the one equation out of hundreds that was the correct one. ...Interestingly, the correct equation was actually considered briefly, but rejected by Einstein because he came to the mistaken conclusion that it violated causality. Then a further mistake led to the abandonment of the principle of covariance."