Boris D. El'konin

Objective. Generalization of the author’s clinical observations, experimental materials and theoretical assumptions over the last 30 years of research in the field of developmental psychology; an attempt to highlight the key sought-for and reconstruct the field of possibilities in the cultural-historical psychology understood as the psychology of development. 

Results. The article gives a generalization of experimental studies and clinical observations of the situation of mediation and the act of mediation. It is substantiated that mediation is revealed. It is presented as an event of cumulative intermediary action. Intermediary action is appropriate in a situation of discovering and overcoming a hitherto implicit boundary of activity. 

Conclusion. It is demonstrated that intermediary action leads to three outcomes: its result is achieving what is needed; its product is discovering the meaning of the sign as a symbol of the space of action possibilities; its effect is procreating the actors’ subjectivity. Intermediary action is a unit of development.