  Before developing his 1915 General Theory of Relativity, Einstein held the
"Entwurf" theory. Tullio Levi-Civita from Padua, one of the founders of tensor
calculus, objected to a major problematic element in this theory, which
reflected its global problem: its field equations were restricted to an adapted
coordinate system. Einstein proved that his gravitational tensor was a
covariant tensor for adapted coordinate systems. In an exchange of letters and
postcards that began in March 1915 and ended in May 1915, Levi-Civita presented
his objections to Einstein's above proof. Einstein tried to find ways to save
his proof, and found it hard to give it up. Finally Levi-Civita convinced
Einstein about a fault in his arguments. However, only in spring 1916, long
after Einstein had abandoned the 1914 theory, did he finally understand the
main problem with his 1914 gravitational tensor. In autumn 1915 the G\"ottingen
brilliant mathematician David Hilbert found the central flaw in Einstein's 1914
derivation. 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 to
me".
