Here are some things we understand, at least pretty well: planetary orbits, cell division, rainbows, electrical conductivity. Here are some things we don’t understand at all: conscious awareness, knowledge, free will, understanding things. That is, we are, as a species, pretty good at mathematics and science and no good at all at philosophy. Why is this?
Van Inwagen must have realized the irony in his position: we can claim to understand things but not the very understanding by which we do so. We know all sorts of things, except for the small detail that we don’t know what it means to know. This is fine as an observation of fact, but it also seems to point to the futility of trying to separate “science” from “philosophy” and claim the first is successful whereas the second is a failure. All “science” is on this account is a doctrine that grounds itself on naive, operationalist principles and which tries to explain as much as it can on this unexamined and provisional basis. We are pretty good at explaining the causes of rainbows, so long as we don’t ask what we mean by “cause” (!); we have a total theory of the universe, but are totally confused about what theories are. For that matter, our account of the “universe” cannot determine whether it is all things or not (since whatever we mean by universe appears to allow for the possibility of a multiverse). Even if we had a theory of everything, it would only be a something-or-other about something-or-other. It might be a “better” something or other than the one it replaces, and it would certainly give us more power to do stuff, but any ultimate certitude we might feel in pondering it would be an illusion we created by forgetting the naive foundations that it rests on. We think we have certitude, when all we have is the consensus of the forgetful.
The success of science in making consensus presupposes forgetfulness, i.e. a group of people agrees to shelve the discussion of the basis of things and work on something else. Philosophy (or even science so far as it turns itself to the fundamental things) refuses to do this, but the cost of doing so is lack of consensus and therefore of progress. For whatever reason, these diverse goods are incompatible for a human mind, and so we are left to choose which good we will be a part of cultivating. Whatever one says about this choice abstractly, as a concrete choice of an individual human life it is largely determined by particular talents, interests, personality quirks and the idiosyncratic facts of history, and in this sense there is no better or worse option.
There will always we a tendency among those with the consensus to tyrannize over others, and to marginalize all those who fall outside of the consensus. This happens not only in science trying to tyrannize over philosophy (i.e. as the various positivist philosophies from Comte to Daniel Dennett have tried to justify), but also in successful philosophies tyrannizing over others, e.g. Neo-thomism persecuting other Scholastic opinions from Aeterni Patris to Vatican II, or the present attempts of Naturalistic philosophy in the Anglosphere Secular University to marginalize their rivals.
I am not here giving an apologia for skepticism. My claim is not “when you look at the foundations, everything is hopelessly obscure!” The claim is rather that looking into foundations is always a relatively lonely affair, and that it should be. For whatever reason, philosophy is killed by too much consensus. The Kantian contempt for “dogmatism” in metaphysics, or the 1930’s kefluffle about the incompatibility of philosophy and religion is really a muddled account of the incompatibility of philosophy and consensus. One can find real certitudes in philosophy (or, again, science as focused on fundamentals) only among a small group of persons. While there is no logical inconsistency in having everyone believe these certitudes, in practice it always leads to an anti-philosophical atmosphere where all rivals as heretics, and to the widespread aping of various ideas by people who call themselves philosophers but who have no idea what their doctrine is actually based on. Mass consensus makes for philosophical laziness, where the great majority of philosophers think they can refute ideas when in fact all they can do is deliver pre-scripted monologues to caricatured opponents.