Abstract In his memoirs Heisenberg reports on his first encounter with Einstein in April 1926. He had been invited to present his work on quantum mechanics in the prestigious Berlin colloquium and after the talk they walked together to Einstein’s apartment. Einstein began the conversation by criticizing Heisenberg’s postulate that quantum mechanics must be based exclusively on quantities which are in principle observable (Episode 37). Astonished, Heisenberg remarked that he thought, just that had been Einstein’s approach in special relativity when giving up the concept of absolute time, Einstein replied [1]: ‘Probably I did use this kind of reasoning, but it is nonsense all the same [… ] In reality the opposite happens. It is the theory which decides what we can observe.’