This principle (or, theorem) can be generalized. Thus, consider a system S in the form of a closed domain of finite volume in the Euclidean space. And let us further consider the situation where there is a stream of ”equivalent” particles into S (number of particles per time unit) where each particle retains its identity while being in S and eventually - after a finite time - leaves the system irreversibly (i.e. for these particles the system is ”open”). The Figure