The clarifications achieved through ethical discussions of the eighteenth century were largely forgotten during the nineteenth, except by one or two, such as Archbishop Whately, who carried no great weight in the field of Moral Philosophy. But towards the end of the century we find the older and clearer language being spoken again by a moralist who commanded the attention of his fellows, namely Henry Sidgwick.--A.N. Prior (1946) Logic and the Basis of Ethics (Oxford), p. 36.
Prior here notes two phenomena: first, philosophical regress is genuinely possible even without the circumstances political or revolutionary destruction of the institutions of collective and professional memory. (Undeniably, there were serious changes between the eighteen and nineteenth century in Britain, but there was also great deal of political and pedagogical continuity.) Nor did he think that such regress is only possible prior to the modern research university. His favorite examples of philosophical regress come from the writings of E.F. Carritt (largely unfamiliar to me). Second, that even at a given time the market of ideas need not be efficient; who remembers or rediscovers X makes a great deal of difference: not all of us command "the attention" of our peers.
That the market in ideas need not be efficient is also clear from another commitment: that Whately (who was a fine economist, too) ought to have commanded more attention not just from his role of remembering the right things, but from the quality of his thought more generally. This his clear not so much from Prior's praise of Whately as a practitioner and popularizer of "the traditional formal logic which has come to us from Aristotle," (ix)* but from the closing line of his book, "Nor need any more be said in order to the establish the fact that Professor [G.E.] Moore's achievement has not been to work a revolution in Moral Philosophy, but simply to help keep alive, in our own age, the eighteen-century tradition of sanity and logical rigour which Sidgwick (with Huxley the agnostic beside him and Whately the Archbishop behind him) kept alive in his," (107) In particular, Whately's use of the argument from trivialization against Paley is analyzed in detail.
As an aside, the denial of a revolutionary status to Moore's writings in Moral Philosophy is undoubtedly a response to the overblown claims about Moore's role in founding a new kind of philosophy -- analytical philosophy. (Prior, a student of Findlay, who was educated in New Zealand, was not sociologically invested, as many are, in overpraising the founders of analytical philosophy.)
Given that Prior thinks that regress in moral philosophy is possible it is not surprise that Prior thought that revolutions in philosophy and other intellectual inquiries are impossible. While discussing the autonomy of ethics from other scientific enterprises, he cites with care (p.43) the work, The Scope and Method of Political Economy, of J.N. Keynes (Sidgwicks's "younger associate at Cambridge," the father of the economist, although Prior treats J.M. Keynes as a logician not an economist). This work by the elder Keynes lays out (recall) the Kuhnian analysis of scientific revolutions including the movement from immature to mature sciences (without the use of incommensurability thesis).
Prior does not really explain the causes of philosophical regress. He may not have a general theory of philosophical regress: "the tendency to fall into fallacious modes of reasoning is rather like an epidemic that breaks out during war." (26) It's pretty clear -- reading between the lines -- that he thinks that (Kantian or) Hegelian jargon is to blame for some of the lack of clarity of the nineteenth century. But he also thinks that certain kinds of regress can also set the stage for an improvement in theory:
"In Sidgwick there is far more than a revival of old positions; he develops them with a new subtlety, no doubt acquired in the process of threading his way through denser obscurities (in the work of his contemporaries) than any eighteenth-century writer had to cope with; but the positions which he thus develops are the old ones, and quite consciously so." (37)
I don't think that Prior thinks it was a good such (a la cunning of history) as such that Sidgwick had to plow through obscure stuff. But it turns out, perhaps, to have been salutary for Sidgwick in order to develop more fine-grained distinctions that helped diagnose errors in others.
It should be clear from the last few quoted passages, that Prior -- who is one of the greatest historians of philosophy -- tends not to see much novelty. Rather he evaluates other thinkers in light of their commitment to maintain clarity and expose fallacies in moral reasoning in others--"a task which it seems to be necessary to perform anew in every age." (x) While it's an interesting fact that when he wrote these lines, Prior was not yet known as a logician, the use of 'necessary' here points to a larger historical duty (and thus thesis about the nature of learning throughout history). Prior clearly thinks that without great vigilance most of what's done in philosophy is sub-optimal. This is the basis of his praise of Moore: "The unoriginality of Professor Moore, for example, on which I have laid some stress in the following pages...does not alter the fact that he has done more than any other single individual to promote clear thinking on ethical subjects in the present century" (x).
If Prior is right, that regress is always a live possibility and that claims to originality tend to rely on a lack of historical background knowledge (or perhaps party-line propaganda), then it's right to write a history with the tools of logic to praise the restorers of clarity and rigor (and provide detailed analysis of their actions) and thereby provide others a further incentive not to be forgotten by philosophical posterity for good reasons. Of course, the fate of an Archbishop's afterlife is probably better served by higher powers.