The principle of Quantum Mechanics — as well as Quantum Field Theory and String Theory — which says that an observer can never know both the position and velocity of a particle with perfect precision. Specifically, the more certain an observer is of the position, the less certain that observer must be of the velocity, and vice-versa.
Come, you lost Atoms, to your Centre draw,
And be the Eternal Mirror that you saw:
Rays that have wander'd into Darkness wide,
Return and back into your Sun subside.
From Farid al-Din Attar's
The Conference of the Birds