Disclaimer: This is way outside of my wheelhouse. So the chances that this is just misguided are great. But it’s my month to post and I’m answering the editor’s (Half-Assed) Plea for Half-Assedness.
Many moral skeptics appeal to the following metaphysical thesis in supporting their view:
- Metaphysical Naturalism (NAT) – There are only natural properties.
And ethical non-naturalists are committed to the following metaphysical thesis:
- Metaphysical Plurality Thesis (PLU) – In addition to natural properties, there are non-natural properties.
Now, many think that NAT is more plausible than PLU on the grounds of parsimony. I think that this is a mistake.
Too see why, I’ll need to introduce some terminology. Let’s say that a theory, thesis, or view, T, has a positive ontological commitment to Fs if and only if T entails that there are Fs. And let’s say that a theory, thesis, or view, T, has a negative ontological commitment to no Fs if and only if T entails that there are no Fs.
Now, I take it that all those who accept the principle of parsimony hold that positive ontological commitments are, other things being equal, bad—that is, they make a theory less plausible, other things being equal. But there are three possible views concerning negative ontological commitments: (1) they too are bad/negative, (2) they’re neutral, and (3) they’re good/positive. Correspondingly, we have three versions of the principle of parsimony (POP):
- (POP-neg) Other things being equal, if T1 has fewer (positive or negative) ontological commitments than T2, then T1 is more plausible than T2.
- (POP-neu) Other things being equal, if T1 has fewer positive ontological commitments than T2, then T1 is more plausible than T2. But T1’s having fewer negative ontological commitments than T2 makes T1 neither more nor less plausible than T2.
- (POP-pos) Other things being equal, if T1 has either fewer positive ontological commitments or more negative ontological commitments than T2, then T1 is more plausible than T2.
Those who think that NAT is more plausible than PLU must eschew POP-neg. After all, NAT and PLU are on a par in terms of POP-neg: they both have the same number of ontological commitments: two. NAT has a positive ontological commitment to natural properties and a negative ontological commitment to the non-existence of non-natural properties, whereas PLU has a positive ontological commitment to natural properties and a positive ontological commitment to non-natural properties.
But it seems to me that POP-neg is the best of the three. That’s because a theory that has fewer commitments is less open to being falsified than a theory with more commitments. And a negative ontological commitment is just as much a commitment as a positive ontological commitment.
Now, perhaps, some would appeal to the history of science in defense of POP-pos over POP-neg. But I don’t think that the appeal works. What the history of science has shown is that when it comes to explaining events in space and time, those who have done so by expanding their positive ontological commitments have, other things being equal, fared poorly. But for the history of science to support POP-pos, there would have to have been cases where theories that expanded their negative ontological commitments fared better when other things were equal. But there are no such cases—at least, not that I know of. Of course, there were cases were other things were not equal. For instance, a theory that denied the existence of unicorns would be better able to explain the absence of certain types of fossils in the fossil record. But what the supporters of POP-pos would need is some case in the history of science of the following sort: a scientific theory denied that there were, say, Platonic forms even though this didn’t help it to explain anything and didn’t have any other advantage. Subsequently, it was proven that there were no Platonic forms. And because of this, this theory fared better than those theories that were otherwise the same but didn’t have this negative ontological commitment. But there are no such cases. So the history of science doesn’t support POP-pos.
Lastly, POP-pos seems committed to the absence of evidence for non-natural properties being evidence for the view that there are no non-natural properties. But that’s just obviously fallacious, isn’t it?
So I don’t see how people can defend NAT by appeal to parsimony and I don’t see any other reason to favor NAT over PLU. So I wonder why people appeal to NAT in defending moral skepticism. And I wonder why NAT is so popular among philosophers. I can understand being agnostic about whether there are any non-natural properties, but I don’t see the logic in denying non-natural properties on the grounds of parsimony.