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Before developing his 1915 General Theory of Relativity, Einstein held the"Entwurf" theory. Tullio Levi-Civita from Padua, one of the founders of tensorcalculus, objected to a major problematic element in this theory, whichreflected its global problem: its field equations were restricted to an adaptedcoordinate system. Einstein proved that his gravitational tensor was acovariant tensor for adapted coordinate systems. In an exchange of letters andpostcards that began in March 1915 and ended in May 1915, Levi-Civita presentedhis objections to Einstein's above proof. Einstein tried to find ways to savehis proof, and found it hard to give it up. Finally Levi-Civita convincedEinstein about a fault in his arguments. However, only in spring 1916, longafter Einstein had abandoned the 1914 theory, did he finally understand themain problem with his 1914 gravitational tensor. In autumn 1915 the G\"ottingenbrilliant mathematician David Hilbert found the central flaw in Einstein's 1914derivation. On March 30, 1916, Einstein sent to Hilbert a letter admitting,"The error you found in my paper of 1914 has now become completely clear tome".

Einstein the Stubborn: Correspondence between Einstein and Levi-Civita