Ever since Thomas Aquinas's elaboration of Trinitarian theology (Summa theologiae I, pp. 27–43), scholastic theology structures its tract on the Trinity along four basic concepts: procession, relation, person, and mission, in this logical order. By procession is meant the origination of the Son from the Father by way of the intellect (generation) and the origination of the Spirit from the Father and/through the Son by way of the will (spiration). These processions are used to explain how God can be one "substance" or "nature" and yet three "persons" at the same time without self-contradiction. These two processions give rise to four "real" relations in the deity but only three "opposed" relations, which are called "persons." The two missions refer to the visible sending of the Son into the economy of salvation in the Incarnation, and the invisible sending of the Spirit at the Pentecost.
Processions of origination presuppose that there is a real distinction between the principle or source from which the term proceeds (e.g., the father) and the term that originates from the principle or source (e.g. the son). "Real" here is opposed to mental or logical. A logical or mental distinction is one that is created by the mind between two realities but does not exist in fact. For example, one can make a distinction in one's mind between Jesus as the Word of God and as the Son of God, but there is no real distinction between the Word of God and the Son of God because both are the same reality. In this case the relation between the Word of God and the Son of God is only mental.
On the other hand, a real distinction is one that exists in the extramental world and is often based on some kind of action between the two realities. For example, the distinction between father and son is a real distinction, just as that between son and father, that is, a distinction between two real objects based on the fact of generation. Consequently, the relations between father and son are real relations. Note that real relations are always reciprocal, that is, between x and y and between y and x.
In summary, real relations imply three things. First, there must be at least two terms that are related to each other. Secondly, there is the ground or basis for this relation which can be of different kinds, the most obvious of which is origination. Thirdly, there is the relationship itself, which is always double, that links the two terms together. This relationship however does not exist in itself and by itself; to use the language of Aristotle, relation is not a substance but an accident. It does not subsist in itself but exists in another. If x and y are really related to each other, the relationship exists both in x and in y. Relations are to be-in (esse in ).
Esse in and esse ad . There is an important sense in which relations can be said to be not only to be-in (esse in ) but also to be-toward (esse ad ). This is particularly true of relations in human beings. Through relationships human persons are open outward to others, have a dynamic orientation toward them, give themselves to them and receive them in return in love and commitment, and in so doing constitute a community of spouses, parents and children, friends, and citizens. In a true sense, humans become fully persons only in and through these relationships. For humans to be person is to be interpersonal.
Applied to the Trinity, the relations between the Father and the Son, between the Son and the Father, between the Father/Son and Spirit, and between the Spirit and Father/Son are real relations because the relations are based on the two processions of generation and spira tion. The Father's relation to the Son is termed "Fatherhood" (paternity), the Son's relation to the Father "Sonship" (filiation), the relation of the Father and the Son to the Spirit "active spiration," and the Spirit's relation to the Father and the Son "passive spiration." These four divine relations are real.
Of these four real relations scholastic theology points out that only three are "opposing relations" in the sense that they are set over against one another in terms of origination. These are: fatherhood, sonship, and passive spiration. That is, only the Father and the Son face each other, and together they face the Spirit. The active spiration of the Father and the Son, which originates the Spirit (the passive spiration), does not constitute an opposing relation by itself since it is simply the already existing relation between the Father and the Son. Consequently, there are only three "persons" in God.
Despite illuminating analogies between relations in the Trinity and those in humans, there is a fundamental difference between them. As pointed out above, relations in humans exist not in and by themselves but only in another reality (esse in ). They are, in other words, accidents and not substances. Fatherhood does not exist by itself; rather, it is a relation that a person possesses toward another. It is of course an existentially important qualification but does not define what a human person essentially is. Indeed, a person still is human without being a father, and one can cease being a father without ceasing to be human.
In God, however, there is no real distinction between substance and accident, between existence and es sence. God does not have a relation but is the relation. Relations do not exist in God's substance but are identical with the divine substance. Divine relations are therefore said to be subsistent relations. They are the distinct ways in which God is: the divine nature existing in the relation of fatherhood is God the Father, the divine nature existing in the relation of sonship is God the Son, and the divine nature existing in the relation of spiration (as the Father's and Son's mutual gift and love) is God the Spirit. In God relation is pure esse ad, pure facing-each-other, pure being-oriented-toward-each-other, pure self-givingand-receiving-another. Divine persons are persons par excellence, the perfect and asymptotic models of "personality" for human persons.
Many modern theologians emphasize the priority of relations over substance in God and consequently in humans as well. The "one God" refers to the Father and not to the common divine substance. It is the relations between the Father and the Son, and the relations between the Father and the Son and the Spirit that constitute who God is. It is the dynamic interrelationship of the divine persons that Greek and Latin theologians refer to when they speak of divine perichoresis and circumincessio (and somewhat more statically circuminsessio ) respectively (see circumincession). In the beautiful words of Augustine: "They [the three divine persons] are each in each and all in each, and each in all and all in all, and all are one" (De Trinitate 6, 12).
Bibliography: k. rahner, The Trinity, tr. j. donceel (New York 1974). w. j. hill, The Three-Personed God (Washington, DC 1982). t. marsh, The Triune God: A Biblical, Historical, and Theological Study (Mystic, CT 1994). g. o'collins, The Tripersonal God: Understanding and Interpreting the Trinity (New York 1999). c. lacugna, God for Us: The Trinity & Christian Life (San Francisco 1991). e. johnson, She Who Is (New York 1993). d. coffey, Deus Trinitas: The Doctrine of the Triune God (New York 2000).
[p. c. phan]