Skepticism and Existence
The usual forms of skepticism emphasize existence: we don’t know that external objects exist, or that other minds exist. We might be brains in a vat and none of the objects of perception really exist, or the bodies we observe do not contain existing minds. These types of skepticism typically cite hallucination as a real possibility: it can seem to you as if an object is thus and so but there might be no object there at all, or a lifelike body might be just an automaton. But these are not the only conceivable kinds of skepticism: the skeptic might see fit to challenge our ordinary beliefs about other properties of objects, such as color, shape, size, weight, etc. We don’t really know that the things external to our minds have the properties we normally ascribe to them. Maybe things are objectively very different from the way they seem and the way we think of them: perhaps they have different colors and shapes, or lack color and shape altogether. Here the model would be visual illusion not hallucination: the object exists but it is otherwise than it seems. There are real things causing our perceptions but they don’t appear as they really are. The skeptic wants to know how we can rule this out. I think my desk is brown and oblong, but maybe it is yellow and square—how can I be sure this is not the case? For some reason this type of skepticism is not mentioned—what we might call “attribute skepticism” as opposed to “existence skepticism”. This omission is curious and I can see no obvious reason for it; perhaps it simply lacks the drama of skepticism regarding existence. If people never had any experience of hallucination, it might be the preferred form of skepticism; in any case it clearly exists in logical space. In some ways it is more natural than existence skepticism, since it concerns ordinary properties not the peculiar property of existence. I don’t really know the shape and color of (existing) objects.
But there is a further reason to recognize this form of skepticism: it avoids a problem for existence skepticism. The problem is that it is open to the anti-skeptic to maintain that external objects must exist given the nature of perceptual experience. For experiences need causes, so it cannot be that their distal objects fail to exist. Even if the cause is a supervising scientist, there something out there causing one’s experiences, and this is the object of the experience in question. There is an external world under the brain in a vat hypothesis, and this world supplies experiences with external causes. So it may be said that the usual skeptical scenarios fail to establish the possibility of experience without an external world; it might even be said that we see the objects postulated in these scenarios. I might really be seeing electrodes in my brain when I seem to see a table. Whatever is causing my experiences qualifies as their object—what I am seeing. So I do know that the external world exists! I just don’t know what it’s like. But this response is not possible for attribute skepticism: the external object might really lack the properties I attribute to it, while it cannot lack the property of existence. It is relatively easy to satisfy the existence requirement, but it is much harder to justify the claim that the table is really brown and oblong. We all know what would have to be the case for something not to be brown and oblong (e.g. to be yellow and square), but there are so many ways of existing that the external world could satisfy that allow it to be very different from what we suppose. We might just be seeing our brain states, existing things, but we would certainly not be seeing them as they are if we are suffering from massive illusion (seeing neurons as brown oblong tables etc.). So attribute skepticism can easily avoid the response to existence skepticism I have mentioned. It should be added to the skeptic’s arsenal both for reasons of completeness and also for dialectical purposes. Knowing that the external world exists and is perceived is not much comfort to common sense if you can’t know anything about it.
 Similarly, we might allow that other minds must exist given that they are needed to explain behavior, but it is a further question whether they are as we normally suppose. Maybe we need color experiences of some sort to explain discriminative behavior, but the experiences need not coincide with our own, as in a classic color inversion case. Existence skepticism about other minds might be false though attribute skepticism is still true. At any rate, we need to distinguish the two.