While most of us have a less difficult time appreciating the unity of the divine substance, by far the most challenging aspect of Trinitarian theology is reconciling how three persons can at the same time be divine yet possess the same substance, In other words, how can God really be three yet also really be one?
Richard of St. Victor fully appreciates this difficulty, but thinks that the nature of charity-love gives us a way into the mystery, an insight that analogically furnishes not only a reconciliation, but even demonstrates a necessity.
Chapter Four looks into the concept of ‘person,’ that concept of existence which applies to rational beings and is predicated of both humans and God. Since the reality of what being a person entails is immediately evident to our intuition through our experience of being persons, Richard argues that this experience of ours provides the way forward into understanding both how being a ‘person’ can be analogously predicated of God, while at the same time transcending it as it applies to created being.
Existence forms the ground of understanding Richard’s use of person, since he finds in the etymology of the term the notion of possessing existence from oneself (the ‘sistence’ of existence) and the notion of its being originated from another (the ‘ex’ [out of] of existence.)
Returning to the different modes of being expounded in Chapter One, Richard maintains that if charity-love requires another to whom it can be given for it to be the perfection of charity-love at all, then this give-receive movement necessitates that the one who receives charity-love possesses it as coming from another. Under this figure, the fullness of charity-love (and thus existence) is not different or lacking in the one who receives its fullness, but rather it is the relation of possessing charity-love which forms the distinction in the one perfection of charity-love, and thus, by extension of the figure, the distinction in possessing the fullness of the unity of the divine substance.
The finale of this detour into the notion of person is to clarify exactly what he intends by the term: an incommunicable existence. Whatever the persons are, each must possess an existence which is in some way possessed only by himself, or else the distinctions between each would dissolve into the singularity of substance. What remains is to define what might distinguish each person in an incommunicable way: a personal property.