In most scientific writings, the word psychology describes things of a different and often incompatible nature. Sometimes, it refers to an entity, such as the mind, and sometimes to an occurrence, such as an activity. In this paper, it is argued that the psychological subject matter can not be an independent natural object, such as the Aristotle’s composite of matter and form that he regarded as an entity and that instead the concept of event is a better descriptor. Psychological phenomena do not exist by themselves, but as the manifestation of particular properties of certain entities which exist in a Universe in everlasting motion. Since psychological events imply a reciprocal and uninterrupted transaction between living organisms and the environment in which they are embedded, functionalism, as it was first proposed, is the best approach to their study.