Title:           Keith DeRose's "brain in a vat"
Author:          Anonymous
Pastebin link:   http://pastebin.com/3fLXZwxD
First Edit:      Wednesday 16th of May 2012 04:22:48 PM CDT
Last Edit:       Wednesday 16th of May 2012 04:22:48 PM CDT

So, consider an "internal twin" of me.  This twin's life was identical to mine up to
midnight last night.  At that time, our life histories drastically diverge, but not in any way
causes a difference in what our experiences seem like from the inside: Our "internal"
lives are still identical.  At midnight, super-advanced aliens snatched my twin's brain
from his body, placed it in a (human)-brain-sustaining vat, and hooked it up to a super-
advanced computer, that, taking into account the output of the brain that is my twin, gives
it appropriate sensory input.  Meanwhile, we may suppose, I remain a normally embodied
human, with no aliens anywhere around me.  The aliens who snatched my twin's brain
from his body are so advanced that they were able to do so in such a way that did not
impact at all on his experience.  Now it is morning, and I have a conversation with my
wife.  My twin is having identical experiences, and so thinks he is having a conversation
with his wife, but in fact he is not.  (His wife is in fact now, unbeknownst to him, in
shock and mourning over the discovery of his de-brained, dead body.)  The internalist
about justification will hold that my belief that I am having a conversation with my wife
has the same justificatory status as does my twin's analogous belief: either we are both
justified in our belief or both unjustified, and to the same degree.  For what it's worth, the
internalist has always seemed to me to be right about this: It seems to me that such twins
can't differ from one another on the justificatory status of their beliefs: If my belief is
justified, so is my twin's; if his is unjustified, so is mine.  In the case under discussion, I
think both me and my twin are justified in holding the belief in question -- even though
my twin's belief is false.  
What about knowledge?  Since my belief (that I'm having a conversation with my wife) is
true, while my twin's belief is false, even internalists, at least as I construe them, can hold
that one of us (presumably, me) knows the item in question, while the other (presumably,
my twin) doesn't.  For a good test case, we need an example where the beliefs in question
are both true.  So: I am holding a cup of coffee.  My twin also believes he is holding a
cup of coffee, but in fact he isn't.  Because I (correctly) believe I am holding a cup of
coffee, I believe that there is a cup of coffee within 10 feet of my brain.  (If you are not a
fan of "implicit" beliefs, you may suppose that I have just been asked whether there is a
cup of coffee within 10 feet of my brain, and so have considered the matter and have
come to a positive conclusion, and, of course, then, that my twin has had experiences that
make him think that he has just been asked whether there is a cup of coffee within 10 feet
of his brain, and has come to a positive conclusion.)  So my twin also believes there is a
cup of coffee within 10 feet of his brain.  He believes this because he (incorrectly)
believes that he is holding a cup of coffee.  But while he is wrong about the matter of
what he is holding, let us suppose that he turns out to be right about the fact that there is a
cup of coffee within 10 feet of his brain: The aliens who have snatched his brain have
taken up the human practice of drinking cups of coffee, and one of the aliens has
carelessly left a cup of coffee resting right next to the vat that holds the brain of my
internal twin.  So, as it happens, my twin's belief that there is a cup of coffee within 10
feet of his brain is true.  So here we have a pair of "twins" who share a certain belief that
is true in both of their cases.  The internalist will say that either both me and my twin
know that there is a cup of coffee within 10 feet of his brain, or that neither of us knows
that.  Since it seems to me that I do know know that there is a cup of coffee within 10 feet
of my brain, but that my twin doesn't know that of himself, the externalist seems to me to
be right about knowledge.  Knowledge seems to me to crucially involve matters that go
beyond true belief plus purely "internal" issues: there are "external" matters beyond the
truth of the belief in question that matter to whether a belief is a piece of knowledge.  
(Since I think my twin is, like me, justified in believing there is a cup of coffee within 10
feet of his brain, but does not know that this is true, even though it in fact is true, I think
the case of my twin's belief that there is a cup of coffee within 10 feet of his brain is a
"Gettier case," in one common use of that term: It is a case of a justified, true belief that
is nevertheless not a piece of knowledge.)