Reduction and Elimination in Philosophy and the Sciences

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Getting out from Inside: Why the Closure Principle cannot Support External World Scepticism — Guido Melchior
Therefore, the correct argument for external world
scepticism has to have the following internalistic structure:
Premise1: P can only have knowledge about the
external world if she infers it from
knowledge about her own sense data
and from knowledge that she is not a
brain in a vat.
Premise2: P does not know that she is not a
brain in a vat.
Conclusion: Therefore, P does not have any
knowledge of the external world.
According to this version of external world scepticism, the
truth of the premises and of the conclusion essentially
depends on internalistic concepts of justification. If it is
possible that beliefs about the external are evident or externalistically
justified, then the sceptical problem vanishes.
The inadequacy of the closure principle
It is a common view that premise1 of external world scepticism
is essentially based on the closure principle which
states in this context that it is at least possible for a person
to justify that she is not a brain in a vat by inferring it from
any knowledge about the external world. It has been
shown, that premise2 is only true if a person’s beliefs
about the external world can only be justified through inference
from her knowledge that she is not a brain in a vat.
Vicious circles do not lead to justification. Therefore, justification
through inference from knowledge about the external
world to the proposition that P is not a brain in vat implies
that premise2 is false. On the other hand, justification
through inference in the other direction implies that the
closure principle does not hold in the context of scepticism.
If the closure principle holds, then premise2 is false. If
premise2 is true, then the closure principle does not hold.
Hence, premise1 of the sceptical argument can only be
based on argument2, but not on argument1 which involves
the closure principle.
One anti-sceptical strategy is to deny the validity of
the closure principle. It is based on the intuition that the
closure principle supports external world scepticism and
that, therefore, arguments against this principle are
objections against scepticism. Their sceptical opponents,
on the other hand, defend the closure principle in order to
defend external world scepticism. As it has been shown
this anti-sceptical strategy as well as the sceptical replies
are superfluous. If the closure principle holds, then
external world scepticism does not exist.
Literature
Brueckner, Anthony 1994 "The Structure of the Skeptical Argument"
Philosophy and Phenomenological Research 54, 827-835.
Brueckner, Anthony 2004 "Brains in a Vat", in: Edward N. Zalta
(ed.), The Stanford Encyclopedia of Philosophy (Winter 2004 Edition).
Byrne, Alex 2004 "How Hard are the Sceptical Paradoxes?" Noûs
38, 299-325.
Dretske, Fred 1970 "Epistemic Operators "Journal of Philosophy
67, 1007-1023.
Moore, George Edward 1925 "A Defence of Common Sense", in: J.
H. Muirhead (ed.), Contemporary British Philosophy (2nd series),
193-223.
Moore, George Edward 1939 "Proof of an External World", in:
Proceedings of the British Academy 25, 273-300.
Nozick, Robert 1981 Philosophical Explanations, Cambridge: Harvard
University Press.
Pritchard, Duncan 2005 "The Structure of Sceptical Arguments",
The Philosophical Quarterly 55, 37-52.