It Will Never Be Possible By

This passage from Physics and Philosophy (1958) is attributed to Heisenberg on many quote sites, but Wikiquote — citing physicist and historian Abraham Pais's The Genius of Science (2000), p. 24 — identifies it as a statement actually made by Niels Bohr. The fuller context reads: "Any concepts or words which have been formed in the past... can never be sharply defined with respect to their meaning. This is true even of the simplest and most general concepts... Therefore, it will never be possible by pure reason to arrive at some absolute truth." The thought is consistent with both men's documented views on the limits of classical concepts in quantum physics.