At Regensburg and elsewhere, Benedict has carefully made the case that modern rationality is itself dependent upon, and inexplicable apart from, the understanding of reason and the rationality of the world produced by Christianity’s appropriation and development of the Hellenic philosophical tradition. This truth is well understood by Lee Harris, author of Civilization and Its Enemies and The Suicide of Reason. Harris is no particular friend of Christianity, but he understands the boldness and crucial importance of the challenge Benedict is raising to intellectuals of the West. Writing in the Weekly Standard, he says:
In his moving and heroic speech, Joseph Ratzinger has chosen to play the part of Socrates, not giving us dogmatic answers, but stinging us with provocative questions. Shall we abandon the lofty and noble conception of reason for which Socrates gave his life? Shall we delude ourselves into thinking that the life of reason can survive without courage and character? Shall we be content with lives we refuse to examine, because such examination requires us to ask questions for which science can give no definite answer? The destiny of reason will be determined by how we in the modern West answer these questions.Benedict knows that Christian history has had its own experience with the sundering of faith and reason. At Regensburg, he cited the influence of John Duns Scotus (1266–1308) and some of the Protestant Reformers who proposed a Christianity liberated from the philosophical thought that they viewed as alien to Christian faith. In the case of Scotus and others, this leads, he said, “to the image of a capricious God, who is not even bound to truth and goodness. God’s transcendence and otherness are so exalted that our reason, our sense of the true and good, are no longer an authentic mirror of God, whose deepest possibilities remain eternally unattainable and hidden behind his actual decisions.”
I have added the emphasis in those last few sentences, ones that criticize Duns Scotus and others for what philosophers call their "voluntarism" about God. Such voluntarism is what certain Christian thinkers have in common with Islam. Pope Benedict identifies voluntarism as the basis of the idea that God is ultimately not bound by anything we can recognize as reason, by what the Stoics called "Logos"—a term that, not entirely accidentally, St. John the Evangelist used for the pre-existent Christ. That idea in turn leads inevitably to divorcing faith from reason, which is the problem Benedict sees as one shared by both Islam and the modern West.
Theological voluntarism hinges on a very sharp distinction between God's "absolute power" (potentia absoluta; 'PA' for short) and his "ordained power" (potentia ordinata; 'PO' for short). PA is the notion that there is absolutely no limit, not even one of logic, to what God can do, abstracting of course from what he has done; PO is what we can, consistently with logic, say God can do given what he has done. Now surely there must be some sort of distinction between PA and PO. For once it be granted that God is both all-powerful and free, the range of what he can, in general, do must be immeasurably wider than the range of what he can do given what he's already done. But I've never had much use for how voluntarists draw the distinction. It seems to me that if God's PA were what voluntarists say, then nothing sensible can be said about God in himself as distinct from how God has manifested himself. And that in turn implies that there is no explanation, even in principle, for how God has chosen to manifest himself other than the unconditioned decree of his will. Both creation and revelation thus appear arbitrary whether or not they actually are.
The problem I've described is not limited to Islam and certain Western-Christian thinkers. Under the influence partly of the Cappadocian Fathers (fourth century) and mainly of the Pseudo-Dionysius (fifth or sixth century), many Byzantine-Christian thinkers have insisted that God-in-himself is absolutely unknowable and ineffable. Thus, the divine "energies"—i.e., God-as-manifest-in-his-contingent-actions—provide no basis for saying anything informative about the divine "essence"—i.e., God-as he-necessarily-is-in-himself, beyond and behind his energetic manifestations ad extra. For reasons I don't have time to describe, that has not often led Orthodox and Eastern-Catholic thinkers into voluntarism. In my book and the Pope's, that's a good thing. But whether we're talking Byzantine apophaticism, Western voluntarism, or Islam, the result is the same in one respect: in the final analysis, it cannot be said that the God behind revelation is of a nature that is, or even could be, revealed by revelation. In himself, God is absolutely hidden and must remain, for us, Deus absconditus. About such a God, nothing informative can be said—now even by way of analogy drawn from the data of divine revelation itself. Let's call that result 'DA'. It's a result yielded by voluntarism too. But regardless of source, DA has always struck me as both philosophically and theologically absurd.
What's philosophically absurd about DA is, of course, that one cannot produce an argument for it without doing what it claims is impossible: saying something informative about God-in-himself as distinct from God-in-action. What's theologically absurd about it is that, if it were true, then divine revelation would not reveal anything beyond its own content; hence, nothing is revealed other than the revealing, which rules out saying that God reveals himself, in addition to his relationship with us, in revelation. That, I suspect, is the main reason why so many Orthodox, such as the ninth-century patriarch Photius, have objected to the idea that the economic sending of the Spirit by the Son ad extra is a revealed analog of the immanent or theological procession of the Holy Spirit ad intra. If DA, then nothing which is not explicitly said in Scripture and the ante-Nicene Fathers about the procession of the Holy Spirit can be said at all.
Although I have problems with such Catholic theologians as Karl Rahner and Catherine LaCugna, I must agree with them in one respect: God as revealed is God in himself. Thus, the economic or revealed Trinity is the immanent or theological Trinity. That's not to say that how God has chosen to reveal himself, in creation and grace, is the only way he could have done so given what he necessarily is. He didn't have to do anything ad extra at all, and the way he's done it isn't the only we he could have. But whatever God might have done, the reasonability of that love which is God would always shine forth as the revelation of God-in-himself.