Functionalism was the idea the mental processes were useful as functional activities to living creatures in their attempt to maintain and adapt themselves in the world of nature. The mind was seen as a Stream of Consciousness, which accrued experience, but often acted without deliberation. For example, usually in an emergency, people react and then they let their emotions and reflections take hold. This is an excerpt on the Functionalist view of psychology by a student of William James. As you read it, note the breadth of mental activity.
Psychology is primarily concerned with the study of mental activity.
This term is the generic name for such activities as perception, memory, imagination, reasoning, feeling, judgment, and will. The essential features of these various activities can hardly be characterized by a single term, for the mind does various things from time to time. Stated in comprehensive terms, we may say that mental activity is concerned with the acquisition, fixation, retention, organization, and evaluation of experiences, and their subsequent utilization in the guidance of conduct. The type of conduct that reflects mental activity may be termed adaptive or adjustive behavior. Every mental act can thus be studied from three aspects - its adaptive significance, its dependence upon previous experience, and its potential influence upon the future activity of the organism.