2 8 D O C U M E N T 3 J U N E 1 9 2 7 would consider it to be indeed possible that particle trajectories could be defined. But the great simplification that results from statistical quantum mechanics, i.e., that the motion of particles occurs classically, insofar as one can speak of mo- tion at all, would be lost, in my opinion. If I have understood your standpoint correctly, you would happily sacrifice that simplification to maintain the principle of causality. Nevertheless, even according to your point of view the purely statistical determinacy of many experiments could not be changed. Rather, we could console ourselves only by the consideration that, owing to the indeterminacy relation , the principle of causality would re- main without effect for us, but that the Lord God would at least know the position of the particle precisely, and thus the law of causality could retain its validity. I my- self, however, find it rather unattractive to attempt to describe physically more than what one knows from the context of the experiments. But now I won’t plague you any longer with these discussions. Bohr[4] is writing an interesting article about quantum-theoretical principles just now, in which he an- alyzes the wave-mechanical aspect of quantum mechanics in more detail, and in which he has also found a few essential errors in my work.[5] But in conclusion, many hearty thanks once again! Yours sincerely, Werner Heisenberg 3. From Max von Laue[1] Zehlendorf, 11 June 1927 Dear Einstein, You probably already know that Sommerfeld[2] has declined the appointment in Berlin. Under these circumstances, I would gladly be Planck’s successor.[3] The faculty’s report gives the ministry a free hand. Couldn’t you use your influence to see to it that I am named to this post? However, it must be done quickly, before the ministry makes a different decision. Kind regards, your M. Laue e2 r - p 1 q 1 h 