A physical theory usually involves the attempt to explain a certain class of physical phenomena by deducing them as necessary consequences of other phenomena regarded as more primitive and less in need of explanation. These more primitive phenomena may at the time the theory is formulated be undiscovered, so that part of the proof of the correctness of the theory consists in demonstrating the existence of the unknown assumed primitive phenomena. A classic example is the kinetic theory of gases, in which the pressure of a gas is explained as arising from the kinetic reactions of colliding molecules, the reality of which was established only later by the discovery of phenomena such as the brownian fluctuations.