How do we decide whether a metaphysical system is the right one or not? Empirical evidence doesn’t seem to be decisive, and given that some metaphysicians have impossible worlds and Meinongian objects it seems anything might happen in such a system. Are all metaphysical issues undecidable—and extending this thought to philosophy generally do you agree with the thought that philosophical questions are also generally without a decisive right answer, that every and any philosophical question can be reopened and rethought?
This is a very hard question. I wouldn’t say that all metaphysical questions are undecidable. I know that I exist, and I think I have very good evidence that I am a material object who is made of other material objects. So I have at least one good reason to believe that compositional nihilism is false. Is this a conclusive argument? It probably won’t convince the diehard compositional nihilist! But that doesn’t mean it is unsound, and it doesn’t mean that I don’t have a reason to believe the conclusion on the basis of that argument. But since I also think that being comes in degrees, there is a further question about how real I am that isn’t settled by this argument. Answering this further question is much harder I think.
The moral I draw from this is that we shouldn’t think that all metaphysical beliefs are epistemically on a par. Some of them are more epistemically justified than others. The question of what in general can justify metaphysical beliefs is also a hard question. (I’m not sure that there is a special problem concerning how to justify metaphysical beliefs as opposed to philosophical beliefs more generally.)
One might be tempted to think that there is endless disagreement among metaphysicians. But that endless disagreement takes place against a backdrop of widespread and mostly unacknowledged agreement. I think that this backdrop has at least two sources.
Some of this agreement might be the result of culpably ignoring alternative views that should be taken seriously, or the philosophical milieu interlocutors are in. Academic philosophy is, for better or worse, subject to fads too, and the increasing arms race to publish might make this worse.
We should diligently look for our philosophical blind spots. The history of philosophy, cross-cultural philosophy, and experimental philosophy can each help with doing this. Attending to disciplines outside of philosophy can help. So can thinking carefully about the space of answers to philosophical questions even when (maybe even especially when) one is not tempted by a specific answer.
But I don’t think the totality of the backdrop of widespread and mostly unacknowledged agreement can be explained wholly by the factors just mentioned. Consider a specific metaphysical question, such as the question of when composition occurs. There are many answers to this question defended in the extant literature. But no one defends the answer that some objects compose a whole if and only if they are in New Jersey. No one defends the answer that some objects compose a whole if and only if there are exactly 15 of them. No one defends the answer that some objects compose a whole if and only if Donald Trump believes that they compose a whole. These answers are nonstarters. They are so clearly false that they don’t even need to be explicitly formulated in order to be rejected. We know that these answers are false. There is widespread, tacit knowledge about what sort of answers to metaphysical questions can be rejected out of hand. Thinking clearly about how we have this knowledge would be worthwhile.
That we can rule out some answers to metaphysical questions fairly decisively doesn’t mean that we can rule in exactly one. With respect to questions in fundamental metaphysics, disciplined and humble speculation might be the best we can hope for.
The whole interview, which covers a good selection of McDaniel’s broad-ranging interests, is here.