Methodological monism is perhaps a term I am coining, and it means that we should generally be reluctant to postulate two or more fundamentally different kinds of entities, worlds, dimensions, or realms. It means we have good reason to strive, in our theory-building, to give an account of the whole of reality (be it mental reality, mathematical reality, moral reality, propositional reality, social reality, aesthetic reality, and so on) in one unified way. Currently, I am trying to articulate the reasons I see for this attitude.
I am not sure what monism implies yet, nor am I sure what dualism or pluralism imply. Is monism, for example, the thesis that there is fundamentally one sort of object, instantiated multiple times, that appears under different guises here and there? Would this imply that electrons, quarks, muons, and photons are fundamentally the same thing, only with different accidental properties or something like that? Conversely, if electrons and quarks and so forth (and their respective fields) were not fundamentally the same thing, would this imply some sort of metaphysical pluralism? (Think of it this way: perhaps the electromagnetic field is just as fundamental as the Higgs field and the gravitational field; neither are they reducible to each other, nor are they manifestations of the same thing, ultimately.)
Another possibility is that, even though electrons and quarks are not the same thing fundamentally, they both share a similar «mode of existence», in some sense. For example, they are both entities with spatio-temporal locations, and they have a two-way causal interaction between them. This would put them under the same «category of being», whatever that means. So monism would be the thesis that there is only one category of being. In the case of physical monism, we wouldn’t have anything that’s not spatio-temporal and that doesn’t have a two-way causal interaction with other spatio-temporal things — even if there were spatio-temporal things that were radically different from one another.
Perhaps in characterizing a mode of existence we can talk about general laws that every entity of that mode or category of being must obey (again, whatever those words mean — attributing names to things we don’t understand gives us the illusion of understanding; that’s why I like when philosophers are silly and they refer to these kinds of mysterious things with terms like “kind of stuff”). Since I have been talking about spatio-temporal things, and thus characterizing physicalism, we might as well say these general laws would be physical laws. (The laws that characterize just the behavior of electrons wouldn’t be general, but perhaps there some that are general, like Schrödinger’s equation. But keep in mind I don’t even know what I’m talking about at this point.)
II. The reasons.
The point is that people should begin assuming monism and trying to defend it; the main reason for this being that it is more parsimonious to postulate one realm than two, three, or twenty-seven realms. This parsimony is important, because I have the intuition that reality would be ontologically parsimonious in just this way. Why would it be constituted of two or more different realms, instead of one big unified thing with features that have very different properties, but are fundamentally “part of the same realm”? (Whatever “being part of the same realm” means — maybe it means everything is fundamentally mental, or that everything is fundamentally spatio-temporal, or that everything is fundamentally the same thing multiply instantiated!)
I have the intuition that it’s more likely that humans are currently too confused in their metaphysical speculation to grasp, while it seems sure that mathematics and mentality exist, how mathematics or mentality could exist in the same way chairs and electrons exist: there seems to be no way for the former phenomena to be at least grounded in the latter phenomena — let alone being of the same kind! (Not to speak of some basic morality!)
Past humans, including humans of great genius, were very tempted to suppose many sorts of supernatural entities and events in order to explain the many aspects of the world we not only didn’t understand, but we couldn’t even imagine some way of understanding. Remind yourself of the distinction Noam Chomsky once made: there are many questions we don’t know the answer to, and they are divided into those we can imagine how an answer would look like (those are called problems) and those we cannot even fathom how an answer could look like (those are called mysteries).
For example, how the hell did language evolve? This is not a complete mystery, certainly it was by well-known mechanisms of mutation and selection, propelled by cultural evolution. It’s a problem, not a mystery. But how does the brain generate consciousness, or how can mathematical statements be objectively true, or how is it objectively wrong to throw acid in an innocent child or steal someone’s hard-earned money, in a world that seems to be be made of brute, non-conscious, non-mathematical, valueless facts? These just seem like mysteries to many people, and I think they are pretty close to being mysteries at this point in time.
Likewise it was for many questions that our ancestors faced: it wasn’t just a problem to answer how and why humans existed, reproduced, and functioned, or how and why it rained and thundered. It was a complete mystery. In situations of mysteriousness, humans just have this natural tendency to suppose some quite different must be going on, something so different that it can’t be the same kind of stuff as ordinary events are.
As such, I think it more epistemically rational to begin supposing there is just one realm, and strongly resisting the temptation to postulate entities of a different realm in order to account for mysteriousness. It just seems too weird and ad hoc that reality would have more than one category of things. (I’ll be frank: at times it seems preposterous that reality is not monistic, and at others it seems it could be the case, though I err more to the former intuitional disposition than to the latter. Metaphysical speculation is frustrating.)
If through reason and evidence we arrive at strong reasons for rejecting monism, we should do it reluctantly. If methodological monism is the most rational course of action, it follows that accepting the existence of God without great reluctance is not warranted, unless God is of the same type of thing as everything else. Following Berkeley, we would have a mental monism.
So I think accepting any sort of metaphysical dualism or pluralism should be done with a sigh of defeat and an air suspiciousness. It should be something we are almost forced to accept, not something we should seek reasons to accept. Doing the latter will throw us into the same unilluminating belief patterns as those of earlier humans, who sought reasons for belief in the supernatural on top of the natural (dualism) and arrived nowhere.
My next two objections are still heavily underdeveloped (as if the first one wasn’t underdeveloped!): firstly, yielding to such dualistic or pluralistic temptations and postulating extra realms usually is not an explanation. No predictions are made when we postulate mathematics is part of an objective realm, for example; this extra ontology does no extra work to us. The same goes for postulating a separate moral realm, whose nature is entirely mysterious; or postulating properties and relations that exist ‘over and above’ the things that have these properties and hold these relations. (This argument is starting to seem rather weak to me, but I take solace in the fact that Putnam pushed forth a similar argument in his Ethics Without Ontology — I suppose I just don’t get his view yet, if I can’t even make a decent case for it.)
If what I’m saying is true — and I don’t know if it is, because I haven’t read or thought much about the philosophy of explanations —, then these postulated entities are usually not serious candidates for abductions, that is, inferences to the best explanation. It would take quite a structured theory of, say, God, to allow us to make predictions of how the world should be if God existed. (I am eager to repeat I am not even sure if this argument is any good.)
Secondly, we have inductive grounds to be very suspicious of such non-monistic attempts, since they have always failed — in fact, the mere fact that we have never concluded something is of a different realm than the one ordinary stuff is made of, might provide probabilistic grounds for concluding, tentatively, that there are is no second or third realm. Sometimes absence of evidence is evidence of absence, when the evidence should (presumably) have been there. This relates to something I have been thinking about God: as an explanatory hypothesis, it has failed again and again, up until it remained only as an explanation of the most fundamental of things. This fact, of course, is no direct argument against the existence of God, but it should make us wary of this hypothesis and compel us to be more skeptical with it and with its appeal to us. It may be appealing just because we have this temptation to postulate ‘magical’ things when we’re faced with a mystery.
This is not to say we should adopt naturalism or physicalism. One can give an account of reality as a coherent whole with an idealist account, that sees reality as fundamentally mental and grounded, say, in the Mind of God, as Bishop Berkeley has done. This is in accordance with methodological monism and with the arguments I have given so far.