Have you heard of Bob-ism? Bob-ism is the belief that Bob’s opinion is the ultimate standard for truth. Neither you nor I am Bob-ists, but how can we argue against Bob-ism? Well, since Bob is only human perhaps we can find some contradictions within his opinions (although it isn’t necessarily the case that Bob will contradict himself). But, unfortunately for us, Bob has decreed that contradictions are irrelevant, and that they don’t undermine the validity of the system. (Maybe Bob is a fan of paraconsistent logic, or, more probably, he might be crazy.) Given that what can we say to a Bob-ist?
Maybe we can accuse the Bob-ist of believing things that contradict experience. Bob is no expert on matters of science, so occasionally he makes proclamations that are factually inaccurate. But the Bob-ist doesn’t care about that, the Bob-ist thinks that Bob is the standard for truth, not experience, and so where the two conflict experience is in error. So those of us who think that true claims must not contradict experience are convinced that Bob-ism is not a good system, but those working under the assumption that Bob-ism is right have no reason to doubt it. Ah, but now the Bob-ists have revealed themselves as irrational and lacking in epistemic virtue, and that gives Bob-ists a reason to reject Bob-ism. Or not, because Bob has declared that the most rational and most epistemically virtuous way to form beliefs is to believe whatever Bob says. And so we are back to where we started, as we understand things Bob-ism is a bad way to form beliefs, but from within the confines of Bob-ism things are reversed, Bob-ism is the best way to form beliefs and it is we who are in error.
Obviously Bob-ism is a fictitious example. But it shows that there may be many of what I shall call systems of truth, systems that define truth, rationality, justification, and so on (or, in other words, systems that say how two sets of beliefs are to be compared to each other), and if these systems are constructed well there may be no way to get from one to the other rationally. There are, of course, irrational ways to get from one to the other. A Bob-ist might, for example, be unable to shake the intuition that experience matters, and thus consider Bob-ism from another point of view. And if that point of view seems better to them (a very subjective and irrational judgment), and paints Bob-ism in a bad light, then they might abandon it for this new system. Obviously things like Bob-ism tend not to survive very long. But there are plenty of systems of truth that allow experience to matter, and even Bob-ism can be made to take it into account (allow Bob’s claims to be true only when they are about non-physical matters), and are thus much more resilient.
And so everyone subscribes to a system of truth for fundamentally less than perfect reasons. I think it usually goes something like this: we develop a bunch of beliefs about the world that we would like to be true, usually including the belief that science/experience is right where we have it. Later, when we start to consider the nature of justification and truth, we tend to adopt the system the best aggress with these beliefs. Maybe not right away, but as we come across new systems of truth we often evaluate our old one from within the new one, and if the new one also happens to support some of the beliefs we would like to consider true then there is strong psychological pressure (possibly unconscious) to adopt it. Of course once we have adopted it then we can justify our choice by appeal to it. And often we do, we go around telling ourselves and others that we have good rational reason to adopt this system of truth over others, because it agrees with experience or is more coherent or whatever suits our fancy. Obviously there is a good deal of self-deception going on in this process; psychologically the way we do philosophy is very messy, and very dishonest, often in ways that we unconsciously hide from ourselves since we place great stock in rationality. I won’t pretend that I am better than this. For the very psychological reasons I have mentioned I cannot properly introspect and determine how I have come to subscribe to the system of truth that I have. But I can reflect on my history and make a good guess. Historically I have always liked science, I’m good at it and science can answer virtually all of our pressing questions about the world (or at least those that seem of most practical importance to me), if we can frame them in the right way. Thus when I considered various systems of truth I was drawn, I suppose, to a kind of naturalism (with several other stops along the way), one that says that all true claims must in some way be justified by experience. And I make room for philosophy by allowing it to be an abstract way of talking about the world of experience.
Although I realize that what caused me to adopt this system was fundamentally irrational no one is in a better position. There is no rational way to decide what counts as rational. But that does not diminish my confidence that it is the right system of truth. Naturally those who disagree with me feel equally confident, and I admit that if they construct their systems with enough care (a great deal of care, to cover all the corner cases) they can be internally consistent, and thus there is nothing that I can say to change their minds. But I take comfort in the fact that in the long run, regardless of who is right, that every theory accepted as true will be one that would be approved by the system of truth I subscribe to. This is because what matters most in the long run is the usefulness of the theory. Consider a naturalist theory about consciousness that identifies it with a certain process versus a dualist theory that says it just happens to co-vary with that process. Eventually someone is going to want to use such a theory to do something with consciousness, to interface with it, to move it around, or to preserve it. For these scientist/engineers the metaphysics of the matter is irrelevant. When they write the manuals and communicate the process they aren’t going to detail a complicated philosophical position about how consciousness merely co-varies with those patterns, they are just going to point out the relevant parts of the brain and say “to alter conscious experience from X to Y implement physical change Z”, which leads to a naturalistic theory about consciousness, because that it is an easier way to deal with the phenomena. This is not wild speculation, this is a simple observation about the progress of science. Non-physical systems and explanations are gradually replaced with physical ones. Eventually everything from Aristotelian physics to Freudian psychology is ground down and replaced with variants that are based solely on experience, leaving out any a priori reasoning, which thus postulate nothing non-physical. To say that some non-physical theory can survive this process is to say that science will be unable to study it (or won’t want to), which seems absurd since I can think of nothing, not even consciousness, which is not already being tackled with a purely physical approach in some branch of science (excepting things like mathematics, logic, and the study of philosophy itself, things that seem fundamentally abstract, and thus not of the right sort to be physical or non-physical).
The fact that other theories will be ground away doesn’t make them wrong necessarily, but it leaves proponents of them in a bit of a bind. Either they can just accept that in the long run they will be replaced by their competitors, whether they like it or not, or they can try to resist the millstone of science. Neither seems like a pleasant situation.