Any process model of problem solving trying to capture goal directed progress needs an evaluation function of the states. Maximation of its gradient at each step serves as a decision rule in chosing among different legally possible moves. The present analysis qives sufficient conditions for the existence of an evaluation of states which is applicable to customary laboratory problems in the study of problem solving. The evaluation, moreover, is shown to establish a foundation for the measurement of "Insight", the anticipatory aspect of human problem solving capacity.