The object of the first part of this paper is to establish the relationships which Leibniz establishes between metaphysical action and dynamic action, in the light of how he elaborates the concept of dynamic action in the texts from the years 1689-90 and in particular of Dynamica de potentia. Once the interdependence of these two notions is revealed, the ambivalence of action can be seen as a means to a new understanding, based on the Dynamics, of the relationships between substance (...) and phenomenon. The correspondence with De Volder is the ideal setting for this clarification. (shrink)