From My Lecture on Human Nature and the New Testament
What is a person?
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’). Thomas views God as ipsum esse or subsistent being.
C. Richard of St. Victor (died 1173) defines "person" (in relation to God) as "an incommunicable existence of the divine nature" (divinae naturae incommunicabilis existentia). 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., 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. Other qualifying properties of persons could include the potential for intentionality (object-aboutness).
E. One difficulty with the Trinity concerns the problem of identity (from Cartwright):
(1) The Father is God.
(2) The Son is God.
(3) The Holy Spirit is God.
(4) The Father is not the Son.
Example of 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, Trinitarians appeal to the concept of relative (rather than absolute) identity. Absolute identity (definition = “X = Y → Y = X”). Relative identity (definition = “X and Y are the same F but not the same G,” where F and G stand for predicates). Hence, the Father or Son are not absolutely identical to “God,” but only relatively identical to the divine substance.
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 putative triunity transcends our phenomenal experiences. 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 (a Kantian approach).