It has been argued that normal everyday utterances about middle-sized objects will be true even if Berkeleyan idealism (or the BIV hypothesis, or any other ultimate ontology) is true.
They believe that they understand the competing theories of metaphysical realism, Berkeleyan idealism, and the BIV hypothesis; but they are wrong.
Consider Putnam's brains in a vat (BIV) hypothesis.
This is partly why the closure-based sceptical argument can be and is often recast as a paradox, where we (putatively) have three individually intuitive claims that seem jointly inconsistent (Cohen, 1988): (a) I know that I have a hand (p), (b) I don't know that I am not a BIV (q), and (c) If I know that p, I know that q.
I come to know, say, that I have hands via a perceptual procedure that doesn't eliminate the BIV alternative since it is irrelevant (given our worldview).
So one can't come to know, say, that one isn't a BIV via deduction from one's knowledge that one has hands via some ordinary (e.g.
And I can know that I have hands via ordinary means but not know that I am not a BIV via deduction from that item of knowledge.
After all, it seems that at least in some cases some people when first faced with some extraordinary possibility (say, the BIV hypothesis) will classify it as "off-the-wall" without that allowing them to rule it out.