'It is certainly interesting to consider to what extent there is a finite universe'.

Einstein reports his latest considerations on gravitation, expanding on his newly-established theory of general relativity: 'In gravitation I am now looking for the boundary conditions at infinity; it is certainly interesting to consider to what extent there is a finite universe, i.e. a universe of naturally defined finite extent, in which all inertia is really relative'. He promises to send 'the detailed paper on gravitation, in which everything is explicitly calculated' ('The Foundation of the General Theory of Relativity', published on 11 May). He has also been working on photochemical equivalence ('I found a neat simplification of the thermodynamic derivation of the photochemical hν-law').

The first part of the letter reports his safe return from Switzerland to Berlin, and refers to a novel in the style of Laurence Sterne on which he and Besso had jointly embarked. Their old friend and mentor Aurel Stodola has come up with a crazy notion ('eine Bieridee') about Brownian motion. Einstein goes on to discuss the movement of fluid flows around a curved surface, providing a diagram of the resulting vortices. Besso has begun lecturing (on patent law at the Zurich Polytechnic), and Einstein refers to his own aversion for lectures: 'one always believes that everything one has to say is obvious. But that is an optical illusion'. Life is quiet and pleasant at the moment: 'I am working at the moment in a very moderate way, so that it is very pleasant, and I am living contemplatively, without any discordant notes'. The director of the observatory in Potsdam, Karl Schwarzschild, has died, of whom Einstein observes bitingly 'He would have been a pearl, if he had been as honest as he was clever'.