 In classical physics, with any physical system is associated a state space, which represents the totality of possible ways of assigning values to the dynamical variables that characterize the state of the system. For systems of a great many degrees of freedom, a complete specification of the state of the system may be unavailable or unwieldy; classical statistical mechanics deals with such a situation by invoking a probability distribution over the state space of the system. A probability distribution that assigns any probability other than one or zero to some physical quantities is regarded as an incomplete specification of the state of the system. In quantum mechanics, things are different. There are no quantum states that assign definite values to all physical quantities, and probabilities are built into the standard formulation of the theory.