I'm behind on a number of things, so the fortnightly book will probably be up tomorrow. But I've been thinking recently on something that I think I might start thinking more clearly about if I actually try writing it down; hence this post.
There is a not-uncommon high-level way of approaching the history of philosophy that follows problematic concepts and attributes all sorts of other problems to them. One popular one in recent decades has been univocity of being; poor Bl. John Duns Scotus gets all sorts of things laid at his feet for that: voluntarism, secularization, you name it. In reality, philosophical ideas don't really work quite like this; there is no guaranteed conservation of concept across philosophers, across schools of thought, or across cultures, and, what is more, concepts and ideas do not have effects in isolation. Terms change meaning over time and potential problems with a concept can be compensated for by other ideas, positions, methods.
Nonetheless, as is often the case, the approach is not wholly unfounded, because concepts and ideas do constrain what you can do. Positions and systems do not arise only out of historical influences but out of logical constraints, as well. What we can do is pick one and, assuming a fair degree of stability, identify the potential issues with it and compare it to how the history actually develops. This ties in to what I've previously called Abstract History of Philosophy.
Stability over long periods of time requires a relatively intuitive idea that can be expected to tie into a fair amount of common ground. So I think if you were going to do this kind of thing, 'univocity' is not a great choice; (1) it's a technical term in a particular field, (2) we know that this field, formal term logic, more or less collapses as a thriving field for several centuries, (3) we know there are several disruptions in how the term was understood and defined (Aquinas, Henry of Ghent, Scotus), etc. I think a better choice would be something like the exclusive distinction of nature and will.
Distinguishing nature and will (or sometimes intellect) as causes is a longstanding practice. It's an intuitive idea; we do, in fact, make lots of distinctions between things made by nature and things made by choice of will. I think there's reason to think, however, that beginning with the late thirteenth century, as time goes on the distinction slowly becomes much more central and significant than it previously had been. And at the same time, it becomes more of a sorting rule: everything increasingly has to get sorted into the 'nature' group or the 'will' group.
If we take this in the abstract, an exclusive distinction between nature and will has a number of obvious implications and imposes a number of constraints on what you can do. It divides the world into two, a logical region of nature-things and a logical region of will-things. Taking the division as exclusive rules out overlaps -- no natural will or volitional nature. What has will would have to be treated as a primitive -- there could no complete natural explanation for will, it's properties, or its actions. Likewise, things that are associated with will -- like purposes -- cannot share a common genus with things that are associated with nature. Things that might otherwise be seen as genera transcending the distinction -- like final causes or activity -- either have to be eliminated as chimeras or knocked onto one side of the distinction.
Something along these lines arguably did happen, for reasons that arguably do relate to the constraints in reasoning imposed by the distinction. The fate of final causes and exemplar causes are both reasonably convincing examples: as the nature/will distinction gets stronger and is treated more generally, they both get evaporated out of the natural realm and confined to the volitional realm. Even people who are very strongly committed to them increasingly locate them exclusively in the mind, which is not at all what had previously been the case, since they would originally have been seen as transcending any such division. It's important to grasp that this doesn't mean that natural finality or exemplarity get refuted; you will search in vain for such a refutation. Rather, the terms just stop being used in a way that ignores the division; even supporters slowly start redefining them to be on one side of the line. 'Final cause' eventually just begins to mean 'purpose'; the notion of immanent finality becomes much more rare, and people think of finality as always indicating something like artificial design; thus we enter the era in which the universe consists of things, undesigned in themselves, that must be explained by the will of God the designer imposing an artificial order on them. But other things get sorted out in similar ways; natural law is increasingly seen less as a natural than as a volitional scheme imposed on nature, ethics increasingly becomes wholly a matter of will and choice, etc. Two different kinds of theories of signs develop; they both inherit a distinction between natural signs and imposed signs. One, however, which we find, for example, in John of St. Thomas (also known as John Poinsot), takes this to be a secondary distinction; the more important distinction is between formal and instrumental signs. This allows for a relatively unified theory of signs; the formal/instrumental division, though defensible in its own right, does not match the natural/artificial distinction. The other, which we find, for example, in Juan Caramuel y Lobkowitz, takes the natural/artificial distinction to be fundamental. This gives a necessarily disunified theory of signs, since natural signs and artificial signs have to be fundamentally different, one being on the side of nature and the other being on the side of will. The latter approach, not the former, endures and spreads and influences people who have never even read the Baroque philosophers who explored it. We could add others.
If nature and will are mutually exclusive, then we seem forced to three general choices for how we understand mind (which is usually seen as will-related) and body (which is usually seen as nature-related): they can either be two completely different things, as in Cartesian dualism, or we can try to eliminate one as an otiose duplication, and thus get either idealism or materialism. If nature and will divide the world, we run into obvious questions about the border between them, and we run into the problem of how freedom of will meshes with unfree nature in our actual experience. Start with an exclusive division of the world between unfree nature and free will, and some kind of Kantianism seems inevitable.
Of course, history is much messier than the ideal. For one thing, archives and libraries exist, and older views keep popping up among people who bother to read old books extensively enough to see that they have options that are no longer considered -- people like, say, the Cambridge Platonists or Leibniz. This is very unsystematic and unpredictable, and even such people are usually interacting with and having to communicate with people who assume the distinction. But philosophical reasoning has to take into account evidence from the natural world and common experience as well -- even if there is sometimes an extensive delay in how it does so -- and assuming a sharp division will inevitably run us into situations in which we don't quite know how the world is supposed to relate to the assumption. Historically, the big problematic fact that stands in the way of easy division into nature and will is Life; we have reasons to put nonhuman animals and plants on the nature side (as the Cartesians do), but there are lots of things about them, especially about the beasts, that seem almost will-like. This helps further the rise of design arguments, as well, and accelerates their dominance, but it also pushes people to try to find other creative solutions to how animal and plant life fits into the equation. You can try to fit it on the nature side, like the mechanists; or you could think that perhaps that we just need to widen the will-side a bit to include vital activities as well as strictly volitional ones; or you could think that perhaps we need a trichotomy, not a dichotomy. The latter two seem to push us into different forms of vitalism. In any of the three cases, though, we are forced to do some stretching and reworking, since life's place in the division is just not immediately obvious once we lose both overlaps and most of our shared features. And of course, this is inevitable; sharp distinctions are not perfectly rigid, and while they impose constraints on problems, they get slowly deformed and reformed because of the problems, as well. At some point it will inevitably get stretched about enough that it is just a completely different thing than it was; but if my students are any indication, we've not reached that point yet.
None of this is definite enough to count as a causal explanation; it's more of a passing of an initial test by a rough hypothesis. It would make a fertile ground for further study.
It should be said that it's also an error to look at the history of philosophy in terms of easy condemnations. I think that the exclusive nature/will division is wrong if taken generally, but if you were to assume it, there are lots of situations in which it simplifies reasoning without any serious harm or makes it easier to investigate a particular aspect of the world. Ideas that last a long time, last because they are handy for things. And, of course, by exploring with an assumption, you can get positions that are very interesting in their own right, which in turn might make some discoveries easier. The human mind is an impressive thing; it can work wonders even with errors, and it is always wrong simply to dismiss a position because of a wrong assumption somewhere. Error doesn't prevent reasoning to genuine truths, especially given that nobody's views are completely erroneous; every so often, error is even a shortcut to new regions of truth that, proceeding from truth to truth alone, would be arduous to reach.
