The triumphant completion of the General Theory of Relativity.

'Read the articles! They provide final release from all the difficulties. The most delightful thing is the exact correspondence of the movement of the perihelion [of Mercury] and general covariance, but the most remarkable is the circumstance that Newton's field theory is already false for equations of the first order ... And now [Max] Planck is beginning to take the whole thing more seriously; though he is still resisting a bit.'. Einstein hints at difficult relations with his other colleagues in Berlin. The letter opens with regrets that he cannot make a planned journey to Zurich because of war-time restrictions: he longs to breathe Swiss air, and to shake off the limitations of wartime Germany which he compares to a 'muzzle'.

In four magisterial papers presented to the Prussian Academy of Sciences in November 1915, Einstein set out the parameters of the General Theory of Relativity: his letter refers in particular to the third of these, 'The Explanation of the Perihelion Motion of Mercury from the General Theory of Relativity'.