I picked these pearls of texts Clark and Nagel:
We are forced wine tasters of the conceivable. Acknowledging
this, I feel condent, nevertheless, that it is inconceivable that I
could now be asleep, dreaming, if no outsider could know my real
environs because in the same boat, for the same reason, because he,
too, could not know he was not asleep, dreaming. Does Descartess
possibility even seem to make sense, if we ask ourselves how the Evil
Demon, or God, could know that he, too, wasn't dreaming and allow
that neither could?
[…] Skepticism frees us from antiquated problems, including itself,
offering us a new, challenging problem.[...] Skepticism leaves us the
problem of the plain, of its structure, the character and source of
its relative “non-objectivity,” and one major tool for unlocking its
secrets, the plain skeptical possibilities. How radically that
structure must differ from the standard type, if capable of permitting
concepts with the characteristics of Dream to be concepts, and the
plain skeptical possibilities to be possibilities, is evident
[...]The certainty that we are not dreaming cannot be justified except
circularly, in terms of those very appearances which are being put in
doubt. It is somewhat far-fetched to suggest I may be dreaming; but
the possibility is only illustrative. It reveals that our claims to
knowledge depend on our not feeling it necessary to exclude certain
incompatible alternatives, and the dreaming possibility or the
total-hallucination possibility are just representatives for limitless
possibilities most of which we cannot even conceive.
I am aware that skepticism about the external world is widely thought
to have been refuted, but 1 have remained convinced of its
irrefutability since being exposed at Berkeley to Thompson Clarke's
largely unpublished ideas on the subject.
In 1971 Nagel is probably referring to the supposed refutation of skepticism by Moore. What Nagel and Clarke are saying is that Moore's arguments only prove that common sense is a possible way of describing the world, and they say that other forms are possible and incompatible. Hence the skeptic instead of asking how I justify that common sense is correct, he ask now how to know which among the possible ways of describing the world is the correct one.
I personally don't like the skeptic's refutation of Moore, but the refutation via semantic externalism.
I don't know Thompson Clarke's references.