Analysis of a range of experimental methods in psychology shows that they can be classified according to the kind of mediation enabling the subject to cope with the task in spite of the limitations imposed by the experiment. Investigation of characteristics of the action, deformation of the action, Piaget's experimental method, the method of dual stimulation and finally the method of stage-by-stage formation of mental actions - this is how methods can be aligned on the basis of the experimental principle of mediation. In this perspective the formation method, unlike "cross-section" methods, is valid because it gives a subject the opportunity of demonstrating his potential.