I made a similar argument as a comment on Scott's blog. Surprisingly, not everybody believed it 8-).
But I think my responses were not that bad [1, 2, 3].
The following is a summary, in case you do not want to click through those comments:
I cannot doubt that I am, however I can doubt that my brain (or any other computational device) exists.
It follows that I cannot be my brain.
Descartes’ argument is really just two statements:
1) There is a function D(x) equal to my doubt about x. Let’s say D=0 means I cannot doubt x and D>0 means I can have doubts or actually have doubts.
2) from D(I) =/= D(B) it follows that I =/= B.
There is an important issue with 1), because my doubts D(x), in general, change over time. However, Descartes’ argument is that such a change in doubt is not possible for I and B: I cannot doubt that I am, but there will always be some doubt about B.
But let us assume you can convince me that I = B, in other words I am my brain. Now let me imagine a hypothetical supercomputer S in the future, which is capable of simulating human brains and in particular exactly emulates my brain B.
Will you also try to convince me that I = S , although it is clear that B is not S?
Notice that this hypothetical supercomputer S is constructed so that every argument b1, b2, … in favor of I = B has an equivalent argument s1, s2, … in favor of I = S.
The only way to avoid a contradiction (I=B and I=S but B is not S) is to argue that S does not really exist, it is only imagined.
But this is exactly Descartes' argument: We reject I = S because we have doubts that S may exist.
added later: After some more back and forth I summarized my argument in this final comment.
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