" Cognitive scientists recognize two types of rationality: instrumental and epistemic. The simplest definition of instrumental rationality-the one that emphasizes most that it is grounded in the practical world-is: Behaving in the world so that you get exactly what you most want, given the resources (physical and mental) available to you. Somewhat more technically, we could characterize instrumental rationality as the optimization of the individual's goal fulfillment. Economists and cognitive scientists have refined the notion of optimization of goal fulfillment into the technical notion of expected utility. The model of rational judgment used by decision scientists is one in which a person chooses options based on which option has the largest expected utility.' One discovery of modern decision science is that if people's preferences follow certain patterns (the so-called axioms of choice) then they are behaving as if they are maximizing utility-they are acting to get what they most want. This is what makes people's degrees of rationality measurable by the experimental methods of cognitive science. The deviation from the optimal choice pattern is an (inverse) measure of the degree of rationality.
The other aspect of rationality studied by cognitive scientists is termed epistemic rationality. This aspect of rationality concerns how well beliefs map onto the actual structure of the world.' The two types of rationality are related. Importantly, a critical aspect of beliefs that enter into instrumental calculations (that is, tacit calculations) is the probabilities of states of affairs in the world. "

― Keith E. Stanovich , What Intelligence Tests Miss: The Psychology of Rational Thought