Thomas McCall acknowledegs in his Which Trinity? Whose Monotheism? that patristic and medieval writers weren’t “fond of such expressions as ‘center of consciousness and will,’” but argues that “it is not at all difficult to find language that coheres well” with such a formulation (238).
He summarizes an argument from Michel Rene Barnes in support: Barnes “recognizes that ‘a knowledge of Gregory’s psychology’ makes clear ‘that personal relationship and consciousness are not important, substantial psychological concepts for Gregory,’ and he holds that for Gregory a hypostasis is ‘an existent with a real and separate existence’ rather than ‘a center of cognition or volition.’”
This doesn’t look helpful to McCall, but he adds: “Barnes also recognizes that Gregory ‘may indeed be said to have a psychology of the Individuals of the Trinity,’ and he makes this case by pointing to Gregory’s discussion of the Holy Spirit: ‘what is at stake is not simple the ‘separate reality’ of the Holy Spirit, but the Spirit’s status as what we would call a ‘person’: the Holy Spirit ‘acts and says such and such things, and defines and is grieved and is angered.’ Barnes thus concludes that ‘there is reason to believe that he understood the need for stronger and clearer language on both the distinct and the personal reality of the Holy Spirit – language which made clear that the Spirit like the Son was a psychological entity with a distinct existence.’ Barnes concludes thus for good reason; as Gregory himself says, the Holy Spirit ‘exists as a person, able to will, self-moved . . . for its every purpose having its power concurrent with its will’” (239).
Gregory’s arguments have been repeated in textbooks for centuries, and indicate a long-standing refusal to limit the definition of “person” to mere subsistence and a tilt toward defining the personhood of the Persons in terms of will and activity.