Some Differences Between Systematic Theology and Historical Theology?

              I had an interesting discussion with my Patristics tutor today (Tuesday) over the difference between historical theology and systematic theology. I asked her what she thought of Richard Cross's argument that Gregory of Nyssa views the divine substance as a universal, i.e., a multiply instantiated (or instantiable, depending on one's theory of attributes) attribute or kind that is, in some sense, identical across its various instances. Her reply, in very short, was that she thought it misleading to read Gregory's writings on the Trinity in such a light, and that reading him in this way was historically anachronistic because it does not take his historical context seriously enough. It's possible I'm misreading my tutor on this point, but I'm not sure if she means that Gregory would not have looked at the divine substance as being either a universal or not a universal or whether she just means such a question is not one he would regard as important. But, either way, it seems to me, he would have to have had some view on the matter.
              By the law of excluded middle, either he thinks it's a universal or he doesn't think it's a universal, whether he thinks the question of its being a universal is important or not or whether it is even appropriate for thinking of it in terms of its being a universal. Perhaps he has not consciously thought on the issue at all; then, in that case, his (other) views on the divine substance will (it seems)  nonetheless commit him to either the view that it is a universal or that it isn't one. So the main question, then, is whether having a view one way or another would have been important to him. In this connection, I don't know Gregory well enough (or the post-Nicaean controversies for that matter) to state an informed opinion. But even if Gregory did not regard the question as an important one, I don't believe (correct me if this is wrong) that Cross even argues that Gregory considers it an important question; he simply tries to get to the bottom of Gregory's ontology.
             And if we regard Gregory as knowledgeable in Greek philosophy, which historians do, it doesn't seem unreasonable to think he had actually thought of the problem in explicit terms, even in passing. And even if we assume that he wasn't well informed or that he didn't think of the question explicitly, his views would still seem to presuppose, willy-nilly, a definite answer to one side of the question or another. It might take some digging to unearth his views, but it would seem wrong to rule out this possibility in an a priori manner.
             And, lastly, even if Gregory did not consider the question an important one, if he did have a view on the matter, and if he is an authority amongst the writers of the early church, this is not something we contemporary Christians should take lightly. For the fact presently is that there is considerable disagreement on this question, viz., debates over whether the Latin theologians had a fundamentally different way of thinking about the Trinity in terms of the three persons viz-a-viz their relationship to the divine substance from the way in which the Greek or Eastern theologians thought of it. Moreover, if one of the main points of doing systematic theology is to uncover what elements of the Christian tradition are serviceable to the church in the present, all the while being careful not to abstract too much from historical concerns, seeing what Gregory and other theologians thought about this and other related or unrelated issues seems like a legitimate enterprise.
           In closing, I should remark that I bear no ill will towards my tutor. I think she's smart and very well-informed on the early church, and I enjoy my discussions with her. And maybe I'm just not understanding her view well enough or representing it fairly. But if I am, this would seem to reflect something of a disagreement between us, though perhaps the difference in our views is not as great as one may think; after all, it's hard to gauge what a person thinks on an issue from just a short reply that they make.