Big guru in Helsinki: Yrjo Engeström
Activity Theory was mainly a result of a larger effort to develop a new psychology based on Marxist philosophy, an effort which started soon after the Russian revolution of 1917. Several programs for restructuring psychology on a Marxist basis were formulated in the 20's and 30's, and very heated debates between proponents of different approaches were not uncommon at that time. One of the first postulates Soviet psychologists agreed upon was the so-called "principle of unity and inseparability of consciousness (i.e., human mind) and activity". The meaning of this principle was that human mind comes to exist, develops, and can only be understood within the context of meaningful, goal-oriented, and socially determined interaction between human beings and their material environment.
Activity Theory is not a "theory" in the strict interpretation of the term. It consists of a set of basic principles which constitute a general conceptual system which can be used as a foundation for more specific theories. These basic principles of Activity Theory include object-orientedness, the dual concepts of internalization/externalization, tool mediation, hierarchical structure of activity, and continuous development. The principle of object-orientedness states that human beings live in a reality which is objective in a broad sense; the things which constitute this reality have not only the properties which are considered objective according to natural sciences but socially/culturally defined properties as well.
Activity Theory differentiates between internal and external activities. The traditional notion of mental processes corresponds to internal activities. Activity Theory emphasizes that internal activities cannot be understood if they are analyzed separately, in isolation from external activities, because there are mutual transformations between these two kinds of activities: internalization and externalization It is the general context of activity (which includes both external and internal components) that determines when and why external activities become internal and vice versa.
The Activity Theory emphasis on social factors and on interaction between agents and their environments explains why the principle of tool mediation plays a central role within the approach. First of all, tools shape the way human beings interact with reality. And, according to the above principle of internalization / externalization, shaping external activities ultimately results in shaping internal ones. Second, tools usually reflect the experiences of other people who have tried to solve similar problems at an earlier time and invented/ modified the tool to make it more efficient. This experience is accumulated in the structural properties of tools (shape, material, etc.) as well as in the knowledge of how the tool should be used. Tools are created and transformed during the development of the activity itself and carry with them a particular culture - the historical remnants from that development. So, the use of tools is a means for the accumulation and transmission of social knowledge. It influences the nature, not only of external behavior, but also of the mental functioning of individuals.