We often know that [concepts] can be applied to a wide range of inner or outer experience, but we practically never know precisely the limits of their applicability. This is true even of the simplest and most general concepts like ‘existence’ and ‘space and time’. Therefore, it will never be possible by pure reason to arrive at some absolute truth.

Werner Heisenberg (1901-1976) German physicist

Physics and Philosophy: The Revolution in Modern Science (1958), lectures at U. of St. Andrews, Scotland (Winter 1955-56)