What We Observe Is Not Nature

This passage from Physics and Philosophy: The Revolution in Modern Science (1958), p. 78 — based on lectures Heisenberg delivered at the University of St. Andrews in 1955–56 — is one of his most precise statements of the epistemological consequences of quantum mechanics. The full sentence reads: "The measuring device has been constructed by the observer, and we have to remember that what we observe is not nature in itself but nature exposed to our method of questioning." The argument draws on the Copenhagen interpretation developed with Niels Bohr: at the quantum level, the experimenter's choice of instrument determines which properties of a particle can be known at all.