It hinges on the distinction, oft employed under various names by Newman, between ampliative and clarificatory inference. As applied to the development of doctrine, said distinction is useful inasmuch as the legitimacy of the latter is much less controversial than the former. Thus, developments that can readily be formulated as merely clarificatory can readily be accepted as legitimate; whereas ampliative developments, which by definition are not merely clarificatory, often are not readily accepted. Orthodox and Anglicans would agree that there are instances of the former; one often cited is the homoousion propunded by Nicaea I, though I am unpersuaded that that formulation was not ampliative. The example Scott spends most of his time on is, relevantly enough, the filioque, especially as argued for by St. Anselm. The questions, of course, are how to draw the line and what place there is for the latter.
Although both questions are largely conditioned if not entirely settled by the teaching authority of the Church, I believe significant theological progress can be made in answering them. The best approach to limning the scope of ampliative inference, I suggest, would be to explore the idea that the process of understanding what the content of divine revelation is—as distinct from "comprehending" that content, which is not really possible for our intellects—recapitulates the unfolding of divine revelation itself. On that score, he's what I wrote in a comment at Pontifications a few months ago, in the course of a discussion very similar to those which have been taking place here lately:
Since the dawn of Christianity, the Jews have argued that Matthew’s quotation of Isaiah 7:14 as evidence for the Virgin Birth is based on a misconstrual of the original Hebrew. While the Septuagint, which Matthew followed, uses parthenos or ‘virgin’, the Hebrew uses alma, which means ‘young woman’ whether virginal or not; and Jews did not go in for imagining virgins having children. Despite countless attempts by Christian apologists to show that Isaiah really meant ‘virgin’—which continue even to this day—it is indeed rather unlikely that the original author actually had that in mind. We can’t really know for sure what he had consciously in mind, but we do know that that is not how he had been understood; for if he had been, then the idea of the Messiah’s virgin birth would have enjoyed much more currency in Jewish thought than it appeared to enjoy in the few centuries before Christ, which was little to none. It would not have seemed a novelty, which is how it seemed to most Jews at the time, who dismissed it accordingly. Yet as Christians we cannot of course agree with the Jews. What we must say is that even if the original human author of Isaiah did not have some future virgin in mind in 7:14, the Holy Spirit most certainly did. That’s why the Septuagint’s creative translation was prophetic. What the original author might have regarded as fantasy, and most of those who took his work as prophecy did indeed regard as fantasy, is logically compatible with what he wrote and is, in point of dogmatic fact, contained “in germ” within it. One could readily multiply examples of all the ways in which NT writers—and later, the Fathers—saw Christ and the Church in the OT even though the OT writers themselves, and certainly most Jews later, would not have seen things that way at all.
The example of Isaiah 7:14, in conjunction with many others, shows that the unfolding of divine revelation as recorded in the two testaments of Scripture is ampliative. So, why couldn't at least part of our assimilation to and understanding of its content also be ampliative? That's what seemed to happen in the New over against the Old Testament; and it might be seen as a corollary of the theological principle that the Christian life "recapitulates" Christ's. If so, the development of doctrine has room for ampliative as well as clarificatory inference.
