Does Putnam's argument beg the question against the skeptic? Bad news for radical skepticism

The most convincing and shortest version of Putnam's argument against the p ossibility of our eternal envattment is due to Crispin Wright (1994). It av oids most of the misunderstandings that have been elicited by Putnam's orig inal presentation of the argument in Reason, Truth and History (1981). But it is still open to the charge of question-begging. True enough, the premis ses of the argument (disquotation and externalism) can be formulated and de fended without presupposing external objects whose existence appears doubtf ul in the light of the very skeptical scenario which Putnam wants to repudi ate. However, the argument is only valid if we add an extra premiss as to t he existence of some external objects. In order to avoid circularity, we sh ould run the argument with external objects which must exist even if we are brains in a vat, e.g. with computers rather than with trees. As long as th e skeptic is engaged in a discussion of the brain-in-a-vat scenario, she sh ould neither deny the existence of computers nor the existence of causal re lations; for if she does, she is in fact denying that we are brains in a va t.