The Atoms Or Elementary Particles Themselves

From Physics and Philosophy (1958), this is one of Heisenberg's most direct statements about the ontological status of quantum objects. Developing a concept he drew from Aristotle's notion of potentia (potentiality), Heisenberg argued that quantum particles do not exist as definite things with fixed properties until measured — they represent a "world of potentialities." The related remark "Atoms are not things" from the same work captures the same idea. His debates with Niels Bohr at Copenhagen in the late 1920s, and with Albert Einstein at the 1927 Solvay Conference, centered precisely on this question of what quantum mechanics implied about the nature of reality.