Abstract
On the basis of a statistical theory it is shown that the internally produced entropy is positive definite in a system which is in interaction with another system when the system goes over from an arbitrary statistical state to thermal equilibrium. Thereafter the most general type of change of statistical states is analyzed which are possible for a system in thermal contact with its surroundings. Thermal contact corresponds to a special type of correlation between the system and its surroundings. It leads to the result that the internally produced entropy increases monotonically with time.