Relevance. The solution of all other problems of scientific psychology depends on the solution of the problem of the nature and evolution of the psyche. In post-Vygotsky activity psychology, the notion of affect was not at the forefront. Objective: To continue the study of the psyche as a form of affective activity begun by Vygotsky. Methodology and methods: The principle of activity and the cultural-historical approach in human psychology. Results of the research: Following Spinoza and Vygotsky, adopting affect as the "alpha and omega" of mental life, the author traces the initial stages of mental evolution in the process of subject activity. Phase analysis of this cyclical process allows us to determine the specificity of mental activity. The psyche is defined as an affect-inducing reflection of activity on its subject. The cellular form of mental activity, affect, is a state of the acting body, due to which the activity potential of the body, i.e. the level of motivation and the assortment of activity schemes, increases or decreases. The simplest affect is desire; in the process of subject activity, desire performs an orientation function and turns into the affects of pleasure or dissatisfaction, the "primary forms of purely mental behavior," according to Vygotsky. Considered from the objective side, in relation to the object of perception, affect acts as an image of the senses. In relation to the subject of perception, the same affect appears as an emotion. As mental activity develops, its cognitive and emotional, and its subject and reflexive sides become differentiated. With regular repetition, action becomes automatic: the pattern of activity is fixed in the structure of the body, and the affective orienting reaction is replaced by the reflexive one. Affects arise and function at the "creative" pole of divided activity – in situations when a new scheme of activity needs to be developed or a previously acquired one needs to be corrected. They signal a change in the external conditions of activity, triggering a search-oriented reaction. The article provides evidence that conditionally-reflex activity is derived from mental activity and gives a critique of the concept of the orienting reflex.