Wolfgang Schwarz

Quiddistic Knowledge, Quiddistic Belief

In their contributions to Lewisian Themes, Rae Langton and Jonathan Schaffer both argue that quidditism -- the claim that possible worlds may differ only in which intrinsic properties play which causal/nomological roles -- does not entail skepticism about intrinsic natures because standard replies to skepticism about the external world carry over to skepticism about intrinsic natures.

But it seems to me that there is an important difference: if quidditism is true, we not only lack knowledge about intrinsic natures, but also any beliefs about them.

I believe that there are neurons in the primate visual cortex that selectively respond to horizontal lines in the visual field; skeptics argue that this belief is unjustified, that I don't really know that these neurons exist. Suppose they are wrong. But consider Fred who has no opinion whatsoever about neurons in the primate visual cortex. It would be very odd to claim that despite that he, too, knows that there are neurons in the primate visual cortex that selectively respond to horizontal lines in the visual field. If quidditism is true, then I believe that with respect to intrinsic natures we are in the situation Fred is with respect to neurons in the visual cortex.

Let w1 be a world where fundamental property F plays the mass role and w2 a world where some other (alien) property G plays that role. Do my beliefs distinguish between w1 and w2? I don't think so.

First, no possible evidence could distinguish between w1 and w2; so if my credences in these possibilities differ, they would have to differ because one of them had higher prior credence than the other. But how could the F-world appear more likely than the G-world? The case is very unlike that of counter-inductive worlds or vat worlds: rationality demands assigning low credence to those, as does leading an ordinary life. But it is hard to see how it could be rational to believe in the F-possibility rather than the G-possibility, and how that could matter for everyday life. In fact, this seems to be a safe case for applying indifference: it would be irrational, one might argue, to assign anything but the same credence to the F- and the G-world. (Things are different for worlds where properties have only partly traded places.)

Second, nothing about my behaviour indicates that I believe in the one possibility rather than the other. If some remote part of this world contained a twin earth where F and G have traded places, I wouldn't notice if, overnight, I were swapped with my twin on that planet. Nor are my states and those of my twin differently responsive to F- and G-perceptions: any external circumstances apt to cause a certain belief in my twin is equally apt to cause the corresponding belief in me.

Third, it is important not to get confused by belief ascriptions. I believe that some things have mass. If "mass" rigidly denotes whatever occupies the mass role, say F, then "some things have mass" is true at exactly those worlds where some things have F. So, one might conclude, what I believe rules out the G world. But that's an unhelpful way of describing things: by this reasoning, anyone who believes that the actual Prime Minister of Liechtenstein rules a small country believes that Otmar Hasler rules a small country. What's true is that I identify mass by its role; the belief I express by saying that some things have mass is that some things have whatever occupies the mass role. That's true in both the F and the G world.

So I think the usual moves against skepticism can't rescue quiddistic knowledge because we don't even have quidditic belief.