A major step forward towards general relativity: 'Now I am completely satisfied and have no further doubt about the correctness of the whole system, whether the observation of the solar eclipse succeeds or not. The logic of the thing is too obvious'.

Einstein (as so often) opens with apologies for his silence: 'I have been working like a wild beast, and what's best, with great success'. He goes on to explain 'the novelty in the gravitation theory', setting out the field equations and their derivation through the 'conservation law', and going on 'I have been able to prove by a simple calculation that the gravitation equations are valid for every reference system that is adapted for this condition. From this it follows that there are acceleration transformations of multiple kinds that transform the equations in themselves (e.g. also rotation), so that the equivalence hypothesis is preserved in its most original form ... // I had, I believe, already proved the strict equivalence of inert mass and weight, and also of the gravitational field, at the time of your visit. // Now I am completely satisfied and have no further doubt about the correctness of the whole system, whether the observation of the solar eclipse succeeds or not. The logic of the thing is too obvious'.

The letter also sends news of the family's impending departure from Zurich (after Einstein's appointment at the Kaiser Wilhelm Institute in Berlin), although Mileva is going first to Locarno with the children, to help their younger child Eduard recover from a bout of serious illness. Einstein will be living in Dahlem, with a room in Fritz Haber's institute: 'I am not at the moment particularly keen to work, for I had to rack myself horribly in order to discover the matter I've just mentioned. The general theory of invariants was only an obstruction. The direct route turned out to be the only feasible one. It is only difficult to grasp why I had to grope around for so long before I found what was so close at hand'. Mileva's postscript describes Eduard's illness, which left him bed-ridden for seven weeks, and mentions her sense of foreboding at the prospect of the move to Berlin.