Scott writes (emphasis added):
My claim in "Further Notes" was that doctrine only develops in accordance with deductive principles. I confess that at least part of the motivation behind my claim was political in an ecumenical sense: I want to set at ease the hearts and minds of those Orthodox who worry that the principle of the development of doctrine warrants introducing radically new (and possibly heretical) belief-statements into the corpus of beliefs that must be held de fide. I think that this is a "valid" worry (non-technical use of "valid" here), and it is one that I share. The difficulty with any and all inductive inferences is that they are subject to (often massive) underdetermination, that is, the evidence can never establish the truth of any particular inference to the exclusion of all competing, non-consistent inferences. This is not a situation in which we want to find ourselves when trying to discover what must be believed de fide.
Scott's motivation here is admirable, and developing doctrine by deduction is certainly preferable if and when possible. That is partly why, a bit further down, he also says about my homoosious example:
...I think that the homoousios doctrine does follow by strict deduction. This is not to say that every premise needed for that deduction is made explicit in Scripture; some premises are themselves intermediary conclusions of other deductions. But the inference itself needs to be deductive or else there is no rationally compelling reason to believe it.
The notion of compulsion here is extremely important--it is not just a rhetorical nicety to stick in a word like that. Inductive inferences have a certain rational warrant to them--that is, if they are strong it is not irrational to accept them--but precisely because of underdetermination there is no compulsion to believe them--we may always question any inductive inference and in so doing we still act rationally. It would be irrational, by contrast, to question a sound deductive inference.
This is where I disagree with Scott. He believes that DD by "non-ampliative inference," in the form of objectively sound, deductive arguments that are thus "rationally compelling," is the only kind admissible for yielding de fide doctrines as a process of development. What's more, there are such arguments, such as for the homoosious.
But does it follow that the Arians as such, and indeed heretics in general and as such, were or are being irrational, in the same sort of way in which somebody with the mental capacity to understand the proof of the Pythagorean Theorem is being irrational if they remain unpersuaded of the theorem by the proof? Hardly. And, sensibly enough, Scott doesn't say it follows.
To be sure, people sometimes are irrational in that sort of way. Anybody who's taught a class, especially in logic or mathematics, would surely agree. Not all rationally compelling arguments actually do compel everybody to accept their conclusions; people sometimes just are irrational. But it cannot be seriously argued that the Arians in general were being like that for decades and generations. They weren't all just being thick, or ornery, or guilty of "bad faith," i.e., of willfully refusing to be compelled to orthodoxy by a rationally compelling argument. Many if not most of them just interpreted the normative sources, which supplied the premises for Nicaea's definition, differently from the bishops at that council. And the point holds quite generally for doctrines that either were or are widely disputed for a long time. That is why I have often said, on blogs where Catholic-Orthodox dialogue occurs, that there are no rationally compelling arguments, using premises both Orthodox and Catholics would accept, from which the truth of doctrines distinctive of Catholicism but questioned or rejected by Orthodoxy would follow. If there were such arguments, the schism probably would not have occurred and certainly would not have lasted as long as it has. While thickheadedness, orneriness, and bad faith can be found in Orthodoxy—as they can be anywhere else—their incidence hardly suffices to explain why the Orthodox are disinclined to restore full communion with the Catholic Church. It would be most unecumenical, as well as false, to maintain otherwise.
What does suffice is honest, informed disagreement about what the normative sources mean. But that is precisely the problem I believe is addressed by claiming that, in some cases at least, DD takes place by ampliative inference. For the meaning of the normative sources is precisely what is supposed to be brought out by ampliative DD; if DD in any form told us anything that is not at least materially present in the deposit of faith, as contained in the normative sources, it would be addition to the deposit of faith and thus unacceptable. The whole point of DD is to make formally clear what is materially present in the sources but is, to some degree, merely implicit.
Some philosophers have thought that that's just what deductive arguments do. Scott seems to think so. But I think the two cases are different. Of course I agree that, if and when DD by ampliative inference is done well, it is not hard to go on to construct a rationally compelling deductive argument for the relevant conclusion. But that is not terribly interesting and not the real job DD is supposed to do. The real job, one might say, is to come up with the right premises for such an argument. That job is one of interpretation, and I agree with Scott that interpretation is not the same thing as inference. So it is tempting to say, as Scott implies, that interpretation rather than inference is what I'm really about when advocating ampliative "inference" in DD. But that would be a false dichotomy. What I suggest is done by "ampliative" DD is interpretation by inference: the conclusions of warranted ampliative inferences in DD just are what the normative sources, which supply the premises for deductive arguments, mean. "Rationally compelling" deductive argument, if and when given, is just icing on the cake.
Philosophers of science acknowledge something called "inference to the best explanation," which is also called abduction. As Scott points out, that is induction by another name; and even when rationally warranted, abduction is not rationally compelling. The most it can do is come up with an explanation that is more plausible than competing ones. Ampliative inference in DD ('AIDD' for short) seems to be much more like that than like either deduction or other, garden-variety instances of induction that are recognized as warranted. For reasons I've already given, I don't have a problem with that; when the subject matter is revealed mysteries, one cannot always or even often expect more. Nonetheless, while ampliative inference in DD is very similar in form to abduction, there are two key differences between them.
One is that abduction typically seeks to explain states of affairs, events, or patterns thereof which are observable. When abductions can be tested, they are tested by further observation. AIDD is not like that. Its explananda are truths to be found in sources such as Scripture, creeds, liturgical expressions, patristic texts, the example of saints, and the experience of the faithful; while texts, liturgies, and people are observable, they are observable only as media and tokens, not as the living doctrines embodied thereby; and it is the latter which constitute the explananda. Correspondingly, the explanantia of AIDD are inferences, drawn from a variety of things, which yield interpretations of those doctrines that in turn can form the premises of deductive arguments.
The other difference between abduction and AIDD is that the latter is useful for telling us what we should believe by divine faith, not merely what we are rationally warranted in believing. The subject matter of AIDD is, after all, the deposit of faith. But absent some underlying principle of doctrinal authority, the inferences involved in AIDD do not, of themselves, give us anything more than rationally warranted interpretations of the normative sources. And so AIDD tells us what we should believe only when its conclusions are certified by authority. Scott seems to recognize this in his final paragraph.
All the same, I am given pause by one criticism he offers. Referring to how I used the example of Isaiah 7:14's history of interpretation, he says: ...the fact that this pattern is to be found in the "unfolding of divine revelation" is insufficient to show that the pattern of inference involved in the development of doctrine is not deductive. I would have to agree with that. Yet I don't think I need offer that pattern in order to show that inference in DD is "not deductive." Some inference in DD may well be deductive; and when we properly interpret the sources, we thereby supply the premises for sound deductive arguments. All I would suggest is that AIDD is often the way we must interpret the sources.