De Broglie and Bohm's causal interpretation of quantum mechanics was later extended by Bohm, Vigier, Hiley, Valentini and others to include stochastic properties. Bohm and other physicists, including Valentini, view the Born rule linking formula_93 to the probability density function formula_94 as representing not a basic law, but a result of a system having reached "quantum equilibrium" during the course of the time development under the Schrödinger equation. It can be shown that, once an equilibrium has been reached, the system remains in such equilibrium over the course of its further evolution: this follows from the continuity equation associated with the Schrödinger evolution of formula_25. It is less straightforward to demonstrate whether and how such an equilibrium is reached in the first place.