The science of logic studies the rules the mind should follow to arrive at truth. Logic is distinguished from psychology by the extent of its domain, for logic is concerned with but one specific category of states of consciousness – intelligence – while psychology studies all aspects of the self. The perspectives of the two sciences are also different. The only goal of psychology is to understand the nature of the mind, while logic studies the mind with the more practical aim of helping us reach the truth. Psychology describes the mind as it is, logic how the mind should be used to achieve the goals of science.

While logic is thus distinct from psychology, they nevertheless remain closely related. First, both are concerned with the nature of man. While logic applies the findings of psychology to a particular end, psychology, as a theoretical science, necessarily precedes its practical counterpart. We must understand intelligence before deciding how it's to be used.

Second, intelligence isn't an isolated faculty capable of acting on its own but rather always acts in concert with the other faculties – will and sensibility, for example, also play a role in religion. Here again, psychology must precede logic.

To some extent, however, psychology presupposes logic, for the latter deals with the theory of certitude, which is the foundation of every science.