To understand the change he is basic to understand the concepts of act and power. Beacon Journal may not feel the same. Act in the conception of Regis de Jolivet is ' ' everything what he is ' ' (JOLIVET, 1972, P. 221), the act in its definition is the determined one, that is, something that is concrete, not only in the material dimension, but while to be current. Already the power is everything what it can come to be, that is, is the possibility or the determinative one. The tree, for example, is a being in act, because he is something determined, however, in the being of the tree exists beings in power, as for example, to be window, to be chair, being picture, that whose nature already this definitive one, even so not in absolute way. In the tree it has the being in power of the window, however, does not exist the being in power of the bird, therefore, in the power already this determining to the natures that of it are possible of if becoming in act.

The beings exist accurately because of this relation between act and power, is therefore that Jolivet says: ' ' the act and the power are not separate beings, but, only principles of being ' ' (JOLIVET, 1972, P. 221), and it is to break of these two principles that the finite, contingent beings if originate, therefore, the finite beings if find in act, however this is limited by the power, in case that they were not they would leave of being finite and they would start to be pure act, depending only on itself. The power possesss some divisions, the active, whose characteristic is the action, that is, the possibility to act in one another one ' ' corpo' ' . Jolivet of as example ' ' the fire that this in harnesses of esquentar ' ' (JOLIVET, 1972, P.