The article was written in the context of creative legacy of A.A. Leontiev (1936—2004). Responding to the scientist’s call to intensify psychological methodology work and enhance “the criticism of notions”, the author analyzes the history and development of the notion of “functional organ” in the psychology of activity. Contrary to the popular recent trend defining mentality (psyche, mind) as a functional organ of individual or soul, the author insists on defining mentality (psyche, mind) as a functional organ of activity, which corresponds rather to the spirit than to the letter of activity approach in psychology by interpretation of A.N. Leontiev School . Such understanding may help to bring psychology out of dichotomy of mechanistic and spiritualistic theories. Even L.S. Vygotsky used to note that without overcoming this barrier the normal development of psychological science and practice is impossible.