A student expresses his/her scientific knowledge simultaneously through his/her way of acting in situation (operative form), and through statements and explanations that she/he is able to communicate (predicative form). The sense is in the activity that she/he develops and not only in the liguistic forms that he/she enunciates. The concept of didactic situation goes together with that of activity in situation, and more precisely with the concept of "scheme". Important conceptualizations are contained in the schemes. The perspective of cognitive development, inherited from Piaget and from Vygotski, is an indispensable reference to continue analyzing in the long and medium terms, the filiations and breaks. But the place of the scientific contents and its own epistemologies it's more important than what these two authors have recognized. Examples are given particularly in the field of additive structures.