Space is blue and birds fly through it (a.k.a. Relational quantum mechanics)

“We were on a walk and somehow began to talk about space. I had just read Weyl’s book Space, Time and Matter, and under its influence was proud to declare that space was simply the field of linear operations.

‘Nonsense,’ said Heisenberg, ‘space is blue and birds fly through it.’

This may sound naive, but I knew him well enough by that time to fully understand the rebuke. What he meant was that it was dangerous for a physicist to describe Nature in terms of idealised abstractions too far removed from the evidence of actual observation. In fact, it was just by avoiding this danger in the previous description of atomic phenomena that he was able to arrive at his great creation of quantum mechanics. In celebrating the fiftieth anniversary of this achievement, we are vastly indebted to the men who brought it about: not only for having provided us with a most powerful tool but also, and even more significant, for a deeper insight into our conception of reality.”