Psychology, or the psychological sciences, refers to a broad array of disciplines that take a scientific approach to mental processes and behavior. Unlike the biological and physical sciences, the psychological sciences are pre-paradigmatic, meaning that there is a lack of a shared language and conceptual foundation that psychological scientists share. Instead of a unified frame, there are a wide variety of schools of thought, such as psychoanalysis, behaviorism, humanism, cognitive science, and so forth, that offer competing versions of the discipline. There isn't even basic agreement about its subject matter, with some schools claiming it is the mind, some consciousness, some behavior, some human behavior. The lack of conceptual coherence can be called "the Problem of Psychology".

The ToK System is intimately related to the problem of psychology. Indeed, the system emerged following a detailed analysis of the philosophical and theoretical problems associated with the discipline. With its depiction of nature as four emergent dimensions of complexity, the ToK System shows how the science of psychology has traditionally spanned two separate but intimately related problems: (1) the problem of animal behavior, and (2) the problem of human behavior. Accordingly, the solution offered divides the field into two broad, logically consistent domains. The first domain is psychological formalism, which is defined as the science of mind, corresponds to animal behavior, and consists of the basic psychological sciences. The second domain is human psychology, which is defined as the science of human behavior at the individual level and is proposed as a hybrid that exists between psychological formalism and the social sciences.