(My actual view (or the view I incline towards, anyway) is that the arguments for and against each of these ontological disputes are equipollent, and that we should therefore suspend judgment about them. My goal in these threads, really, is usually either inquiry or to make sure people are engaging in proper dialectics.)

I distinguish Moorean ontology from analytic ontology. I accept that the car speeding towards me as I cross the street exists; I, however, suspend judgment as to whether the car should be assayed as bundles of tropes, or thin particulars and universals, or nominalist blobs, or whatever. Similarly, I accept that there are minds; I, however, suspend judgment as to whether those minds are immaterial substances, or reducible to the physical, or whatever. Basically, I accept all Moorean facts, but become skeptical whenever we try to analyze those Moorean facts. (See here for links to discussions of the idea behind “Moorean facts”.)

Here is an argument for my skeptical thesis. (i) the disputes over every substantive ontological thesis are rationally undecided (in the sense discussed here); (ii) the principle of disagreement holds; therefore (iii), we should suspend judgment over those theses. Now, even if it turns out that I'm wrong about some ontological thesis, I think the argument pretty clearly succeeds for vast swathes of ontology. (So much, in fact, that I would no longer be confident that I'm not just missing something about whatever thesis appears to make it through.)

Hypatia wrote:

I see. My background is actually existentialist, which means that I am not a pure rationalist, so it's #3 I would most strongly dispute. Not least because it has a certain normative element. I'd certainly consider the quintessential problem of free will to be rationally undecided, but to suspend judgment on it is going to put you in a psychological limbo that will have consequences both personally and societally. Not to get really Sartrean, but I make ontological judgments because I do lean more in one direction than the other, and it would ultimately be an act of bad faith to remain neutral under false pretences.

I see. Would you say that there are certain philosophical issues that we simply have to take an attitude towards, whether they're rationally decided or not, or do you mean something else—more ethical, perhaps?

(I should probably have mentioned that the principle of disagreement's “ought not” is epistemic: “It is not rational, or warranted, or justifiable. . .”.)

Depending on what you mean, I think he's more likely to attack the idea of a unified self—to at least try to shake our confidence in it. Something tells me Derek Parfit has famous work on this, but it has been a while.

But materialists can get many forms of unity by solving the general problem of unity. For instance, if the various parts of a thought are identifiable with goings-on in the brain and the brain and its goings-on are unified, then it seems the thought is also unified.

John West wrote:

I see. Would you say that there are certain philosophical issues that we simply have to take an attitude towards, whether they're rationally decided or not, or do you mean something else—more ethical, perhaps?

Honestly, perhaps a bit of both. I think it's difficult to not take an attitude towards most of these philosophical issues--we can know intellectually that Berkeleyan idealism could be correct, but I don't think most sane people are going to be suspending judgment about it in their day to day life.

But if we can't help but take attitudes towards rationally equivalent possibilities, what will ultimately be informing what our default assumptions are? Cultural prejudices? If there's an ethical dimension to the problem, I think this is where it turns up.

John West wrote:

(I should probably have mentioned that the principle of disagreement's “ought not” is epistemic: “It is not rational, or warranted, or justifiable. . .”.)

I would still worry that the Moorean approach is giving an undeserved privilege to certain "unquestionable" facts. What can we say about people who follow contemplative disciplines for decades? Their minds come to work differently and an immaterial mind might be as utterly real to them as the existence of other minds is to us. Why is our belief undefeatable but theirs is not?