The article presents an attempt of explication of the Spinozist basis of the theory of activity in general psychology. Possible reasons for rare references to B. Spinoza by A.N. Leontiev are examined, in comparison to L.S. Vygotsky, who considered it necessary to “revive Spinozism in Marxist psychology”, built, to his mind, on a dualism of Cartesianism. L.S. Vygotsky’s works are analyzed, in which the historical conditioning and methodological limitations of spiritualism and mechanicism were presented as the possible ways of removing the dichotomy in the context of Spinozism. Further, the article states that the definition of activity in the school of A.N. Leontiev as a substance, which has poles – subject and object, and the definition of the mind, correspondingly, as a function of this substance, directly follows from ontological monism by B. Spinoza and its further development in authentic Marxism, where substance started being interpreted as a universal interaction. It is shown that for A.N. Leontiev the starting philosophical category also was interaction, one of the forms of which is activity, studied in psychology in its specific function (orientative in a wide sense); from this the representatives of the A.N. Leontiev’s school concluded that all psychic processes – even the elementary sensations – have the activity nature. It is emphasized that according to the principal statements of this school, mind is to be understood as a function (attribute, in Spinoza’s terms) of activity, and not as its special form. This means that the psychic is not equated to the “internal activity”: it acts as a functional organ of the “external” (practical) and “internal” (theoretical) activity, which performs its orientative-regulative work in both of them. Such a solution of the mind-activity problem allows removing the dichotomy of spiritualism and mechanicism and criticizes the popular definition of mind as a function of the brain, which reflects external world per se.