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.
Now, here, you see, it takes all the running you can do, to keep in the same place. If you want to get somewhere else, you must run twice as fast as that.
The Red Queen in Lewis Carroll's Through the Looking Glass