Tertullian on the Subject of Eternal Res Et Personae

Concerning Tertullian's fuller statement of God's
existence prior to the generation of His Son, A. Harnack
perspicuously notes that although the ratio et sermo dei
existed within God since "he thought and spoke
inwardly," God the Father was still "the only person"
subsisting prior to the temporal generation of the Son
(Harnack, History of Dogma, 2:259). Edmund Fortman
also concludes that the preeminent Son of God: "was generated, not from
eternity but before and for creation, and then became
a second person." Antecedent to his generation,
however, the Logos was not "clearly and fully
personalized" (Fortman 111). It therefore seems
erroneous to think that the Son was eternally a res et
persona internal beside God. Tertullian makes this
point clearer in Adv Prax 5.

The Carthaginian believes that God the Father was not completely
alone before He created the world since he had his own
ratio within him (Adv Prax 5). Tertullian's main point,
however, appears to be that just as a man reasons
within himself and discourses inwardly, thus making
himself an object of contemplation, so God (from all
eternity past) discoursed and reasoned internally.
Such inward and rational discourse was apropos for The
Most High God (Summus Deus): "For God is rational, and
reason is primarily in him, and thus from him are all
things: and that Reason is his consciousness"
(rationalis enim deus, et ratio in ipso prius, et ita
ab ipso omnia: quae ratio sensus ipsius est). Yet the
reason (ratio) dwelling internally beside God the Father ante creation was not an eternal res et persona as Tertullian goes on to demonstrate.