A quantum-mechanical principle formulated by German physicist Werner Karl Heisenberg. It proposes that measuring either one of two related quantities, such as position and momentum, or energy and time, produces uncertainty in the measurement of the other. The error in measurement of one (for example, position) times the error in measurement of the other (for example, momentum) must be greater or equal to Planck's constant. This principle accounts for bizarre behavior of subatomic particles and implies that we can never know everything exactly.