In
both my recent notes on indifference principles, the comments on Nick Bostrom’s
computer simulation paper and Adam Elga’s Dr Evil paper, I’ve mentioned that
the proponents of these theories assume a theory of evidence that is
intuitively quite plausible, and may have been the mainstream view not long
ago, and may even be ultimately true, but which is not very popular among
philosophers of perception these days. I didn’t think much followed from this,
save perhaps that those presupposing a theory that is widely viewed as being
hopelessly befuddled owe us an explanation as to why they are sticking with it.
And in this little endeavour I have been utterly unsuccessful. This could be
because my heart hasn’t really been in it due to underlying internalist sympathies,
or because I’m wrong that the indifferentists need to address this, or because
I’m no good at convincing people of things, or because of any number of other
reasons. Suffice to say that in some circles, the idea that when we look at a
hand we have evidence of an epistemically different kind to a brain-in-a-vat
that is stimulated in the way our brains are when we look at a hand is not
viewed as being particular plausible.

When
in trouble in a case like this, call in the heavy hitters. Alex Byrne has a paper
forthcoming in Noûs in which he
argues that the sceptical
paradoxes are not really deep paradoxes. By this he means, in part, that
there isn’t anything like a compelling argument for scepticism. And this is because
he thinks that the canonical arguments for scepticism turn out to rest on very
implausible premises on close inspection. One of those premises is that
perceptual evidence underdetermines what the external world is like: we could
have just this evidence and be dreaming (or a brain-in-a-vat, etc.). This,
Byrne thinks, can be shown to be false simply by carefully reflecting on the nature
of evidence. The whole paper is worth reading, but let me just extract a few
choice quotes.

          The
known (evidence) proposition e has yet to be identified. [Byrne has just
argued that evidence should be propositional. The challenge is to determine
whether there is any candidate to be e that is compatible with
thorough-going external world scepticism.]The candidates may be divided into
two classes. The first—class I— consists of propositions about S’s sense-data,
ideas, impressions, phantasms or other queer entities allegedly “given” in
experience. The second—class II—consists of propositions about how things look
or (visually) appear to S (cf. the first paragraph of this section [not
excerpted here.]).

          It
is quite doubtful that (trivial exceptions aside) any propositions in class I
are true, a fortiori known; they may accordingly be dismissed. This
would have sounded dogmatic as recently as the first half of the twentieth
century: it is only in the last fifty years or so that the deep flaws in what
used to be called the “representative theory of perception” have become
gradually visible. Admittedly, not everyone agrees that the theory rests on a
soggy bog of error: in one form or another, it still has its defenders. However,
it is unnecessary here to rehash the argument: because we are playing the first
sceptical game, the sceptic must steer clear of philosophical controversy.

          That
leaves the members of class II: propositions about how things look or appear to
S—in other words, certain propositions about S’s mental states.
But because the representative theory of perception is off-limits, there is
very little motivation for thinking that one’s knowledge of the external world
rests on a foundation of knowledge about one’s own psychology…

          Propositions
about how things look or appear to S can be divided into two types. The
first—type II_E—comprises external world propositions,
because they entail the existence of o: that o looks square to S,
that it appears to S that o is square, etc. Hence, propositions
of type II_E, despite not entailing p, and perhaps being
known by S, are quite unsuitable candidates to be e. For e is
not supposed to be an external world proposition.

          The
second—type II_I—comprises those propositions about how
things look or appear to S that are not external world propositions (or
so we may suppose): that it appears to S that (some x) x is
square, that it appears to S that the F is square (for various fillings
for ‘F’, e.g. ‘tile’, ‘pink thing’), etc. If e is to be found in
class II, it must be of type II_I.

          [I]t
is not plausible that e is a type II_I proposition.
First, these propositions have to be true; clearly we need not suppose
that it appears to S that the tile, or the pink thing, is
square. But is it even clear that it must appears to S that (some x)
x is square? If not, then since there are no better candidates, e is
not a type III proposition. Second, S believes e, and it is
quite unobvious why S, if he is to know p via his senses, must
have any beliefs about how things appear, let alone believe one of the
specific propositions under consideration. Suppose S is a conceptually
challenged animal who cannot entertain these comparatively sophisticated
thoughts about appearances; does this fact alone imply that S cannot use
his eyes to come to know that o is square?