This paper compares the concepts being-in-relation and self-knowledge in Plotinus' V Ennead and in the second part of St. Augustine's De Trinitate. It shows how in both authors being knows itself as intrinsically relational and dynamic, inserted in a process of continual conversion. In both, man's self-knowledge in his intra-psychical relations tantamounts to knowing himself in his relations with God. However, in the neoplatonic, man's relation to God ends up identified with the relations among the Divine hypostasis. In Augustine the trinitarian relations, man discovers in his interior essence, are a created mirror and image, sharply distinct from the Divine Essence, although they don't operate without the necessary vertical relation with the Divine Trinity. Augustine brings to its first sistematization the fundamental pillar that holds together Christian Natural Theology and Philosophical Anthropology.