The article introduces and defends Aristotelian ontological theory of the possible as that which a power (active potency) is capable of bringing about. It regards this conception to be a sort of middle way between Platonic explanation based on abstracta on one hand and the possibilist theory ultimately making everything possible into actual on the other. The doctrine defended leads to the conception of necessary being. Combined with other assumptions concerning this being, there arise some interesting issues and apparent tensions to be resolved. The article outlines some of the discussions related to these problems in medieval and early modern Scholasticism.