Two competitive models are alternatively present in plotinian metaphysics. In the first one, the intelligible ousia appears to be the substrate or matter of its different determinations. In the second, Plotinus abolishes all exteriority between the ousia and its acts, energeiai, which are Forms. In this second perspective, the ousia is the generative principle of its own constitution : any idea of substrate or matter must henceforth be banished from the intelligible realm. The aim of this paper is to shed light on the influence of this second model on the psychology and theology of Augustine. More precisely, the augustinian theory of the mutual immanence of the three divine Persons could have its origin in the plotinan ontology.