I. Trinity Doctrine and Ethics
A. God is supposedly three persons in one substance (i.e. Father, Son and Holy Spirit).
B. The term "person" is used analogically (i.e. God is both like and unlike human persons) when one applies it to God. God does not have a body or God has intuitive rather than discursive knowledge.
II. What "Person" Means when Applied to God
A. The modern conception of person implies a distinct center of consciousness (e.g. Rene Descartes' cogito ergo sum).
B. This usage becomes problematic when one speaks of the divine persons in terms of distinct centers of consciousness. The terminology then implies tritheism (the belief in three gods). Yet, to speak of one center of consciousness obtaining in the triune Godhead implies modalism (= God reveals Godself in three successive modes), not Trinitarianism.
C. In any event, Trinitarians argue that one needs to avoid defining "person" (in this case) as a distinct center of consciousness or rationality. Some other definition must be more suitable.
III. Definitions of "Person" for God
A. Boethius (circa 475-525 CE): "an individual substance of a rational nature" (rationalis naturae individua substantia).
B. Thomas Aquinas (1225-1274) contends that the term "person" when applied to God does refer to "an individual substance of a rational nature" (rationalis naturae individua substantia) as long as one carefully nuances or qualifies what is meant by "individual" (i.e. incommunicable) "rational" (non-discursive, but intellectual) and "substance" ('self-grounded existing'). Aquinas views God as ipsum esse or self-subsistent being.
C. Richard of St. Victor (died 1173) defines "person" (in relation to God) as "an incommunicable existence of the divine nature." Persons have a certain property that distinguishes them from other persons (Fortman, The Triune God, 191-192).
D. Some believe that the Trinity doctrine possibly helps us to understand what personhood entails. Maybe a "person" is an individual substance of a rational nature, one who either actually reasons or who has the potential to deploy reason (i.e. the faculty of inference or intelligence). The term "persons" may also have reference to entities that intelligently relate to one another as Father, Son and Holy Spirit putatively relate to one another in the Godhead.
E. But one difficulty with the Trinity doctrine concerns the problem of identity. For instance, consider the following set of propositions:
(1) The Father is God.
(2) The Son is God.
(3) The Holy Spirit is God.
(4) The Father is not the Son.
Number (4) appears to be inconsistent with propositions 1-3. Let us also consider this example using the planet Venus:
(a) Venus is the morning star.
(b) Venus is the evening star.
(c) The morning star is not the evening star.
To solve this apparent difficulty, certain Trinitarians appeal to the concept of relative (rather than absolute) identity. The definition of absolute identity = "X = Y → Y = X"; relative identity = "X and Y are the same F but not the same G," where F and G are both predicates. Hence, the Father or the Son are not absolutely identical to "God," but only relatively identical to the divine substance. One question remains, however. Does relative identity actually resolve the putative tensions obtaining between the Trinity doctrine and absolute identity?
F. Another seeming difficulty with using the Trinity doctrine to determine what it means to be a person might also be the fact that God's purported triunity seems to transcend our phenomenal experiences (a Kantian argument). Whether God is triune or not appears to be noumenal concern, not a phenomenal one. God's triune nature just might be thinkable but not knowable like Kant's Dinge-an-sich.