Welcome, Ahmad Lotfi. I take it the key text here is Crane's question :
The BIV has no body. But it seems to the BIV as if it had a body. Is this enough to enable the kinds of thought that really having a body involves?
Your question and text connect Descartes's problem withe the evil demon and the brain in a vat example introduced by Gil Harman in Thought, 1973. If I tackle the brain in a vat case first, then we can thread back to Descartes. I am going to suggest that there is a significant difference between the situation of the brain in a vat and Descartes' predicament of possible deception by the evil demon.
Crane's question
The BIV has no body. But it seems to the BIV as if it had a body. Is this enough to enable the kinds of thought that really having a body involves?
Putnam's 'no'
The late Hilary Putnam answered firmly, 'no' : 'on the supposition that we
are brains in a vat, we could not 'say or think we were'. (John Heil, Are We Brains in a Vat?', Canadian Journal of Philosophy, Vol. 17, No. 2 (Jun., 1987), pp. 427-436 : 427.) Quotations from Putnam are taken from Hilary Putnam, Reason, Truth, and History (Cambridge: Cambridge University Press, 1981.
Heil spells out some detail to explain Putnam's view :
The difficulty is that thoughts entertained by
brains in vats lack the right sorts of connection with features of the world
outside themselves, connections required for their thoughts to
represent what they seem to represent - what they would represent were
they present in an ordinary brain. (Heil : 428.)
The key point is that their thoughts do not have 'the right sorts of connection with features of the world' to represent bodies or to enable the kinds of thought that really having a body involves.
This clearly needs expansion. It's just the headline. Heil sets out a fuller statement of Putnam's argument as follows.
Putnam's argument
Putnam begins by envisioning a 'physically possible world' in which
...a human being... has been subjected to an operation by an evil scientist. The person's brain... has been removed from the body and
placed in a vat of nutrients which keeps the brain alive. The nerve endings
have been connected to a super-scientific computer which causes the person whose brain it is to have the illusion
that everything is perfectly normal. (5f.)
Putnam then proceeds to argue both that, on the supposition that we
are brains in a vat, we could not 'say or think we were,' and that this
fact entails that 'we are brains in a vat' is 'necessarily false' (15). Elsewhere Putnam expresses his conclusion differently: 'the brain vat hypothesis turns out to be incoherent' (22). Assuming 'the brain
in the vat hypothesis' is just the contention that we are brains in a vat,
then it appears Putnam takes his argument to show that such a
hypothesis is either necessarily false or incoherent.
It is perhaps unusual to begin the analysis of an argument by
speculating about the character of its conclusion. A charitable reconstruction of Putnam's discussion, however, requires nothing less. The
question is: what sort of conclusion does that discussion support? Is
it, for example, that we are not, or perhaps could not be brains in a vat?
Or is it something less (or more) than this? We are told that 'We are
brains in a vat' is 'necessarily false,' that it is 'incoherent/ but also that
our being brains in a vat is 'physically possible.' What are we to make
of such remarks?
Putnam summarizes the argument as follows: '..[T]he supposition
that we are actually brains in a vat, although it violates no physical
law, and is perfectly consistent with everything we have experienced,
cannot possibly be true. It cannot possibly be true, because it is, in a certain way, self-refuting' (7). One way in which a thesis can be 'self-
refuting' is that 'the supposition that the thesis is entertained or enunciated, implies its falsity" (8) The supposition that we are brains in a vat
is, according to Putnam, like this. 'If we can consider whether it is true
or false, then it is not true.... Hence it is not true' (8).
There is, Putnam supposes, a special difficulty in 'considering' - that
is, entertaining the thought - that one is a brain in a vat. Thus, although
it is 'physically possible' that we are brains in a vat, 'although people
in that possible world can think and "say" any words we can think
and say, they cannot. . .refer to what we can refer to. In particular, they
cannot think or say that they are brains in a vat (even by thinking "we
are brains in a vat")' (8). The difficulty is that thoughts entertained by
brains in vats lack the right sorts of connection with features of the world
outside themselves, connections required for their thoughts to
represent what they seem to represent- what they would represent were
they present in an ordinary brain.
The nub of Putnam's argument is this :
...[T]he fact that [brains in a vat] are conscious and intelligent
does not mean that their words refer to what our words refer....
[There] is no connection between the word "tree" as used by these brains and actual
trees.They would still use the word "tree" just as they do, think just the thoughts they
do, have just the images they have, even if there were no actual trees. Their images,
words, etc., are qualitatively identical with images, words, etc., which do represent
trees in our world; but... qualitative similarity to something which represents an
object... does not make a thing a representation all by itself. In short, the brains in
a vat are not thinking about real trees when they think "there is a tree in front of me"
because there is nothing by virtue of which their thought "tree" represents actual
trees. (12f.)
The point may be extended to brains in a vat thinking of 'brains' and
'vats'. Because such thoughts lack the right sorts of connection with
genuine brains and vats, they are not, despite appearances, thoughts
about brains and vats at all. If they are about anything, they are
(perhaps) about 'images' of brains and vats or goings-on inside the
'super-scientific computer' that gave rise to these images. 'So, if we
are brains in a vat, then the sentence "We are brains in a vat" says
something false (if it says anything). In short, if we are brains in a vat,
then "We are brains in a vat" is false. So it is (necessarily) false' (15).
Imagine an envatted brain thinking 'I am a brain in a vat.' This sen-
tence corresponds, it seems, to the English sentence 'I am an image
of a brain in a vat' or perhaps 'I am an occurrence inside a computing
machine.' If these sentences mean anything, they are surely false: an
envatted brain harboring them is not an image of a brain or an occurrence inside a computing machine, but a brain in a vat.
Full circle back to Crane's question :
The BIV has no body. But it seems to the BIV as if it had a body. Is this enough to enable the kinds of thought that really having a body involves?
The BIV could not have 'the kinds of thought that really having a body involves' because (to amend Putnam's quotation) the brains in a vat are not thinking about real bodies or having the thoughts that really having a body involves, because when they think "I have a body" there is nothing by virtue of which their thought "body" represents actual bodies.
Descartes and the evil demon
It seems to me that if the (putative) evil demon had really persuaded Descartes that he had no body, the case would have been rather different from the predicament Putnam describes. After all, Descartes would really have had a body, despite the evil demon's ability to persuade (or to cause him to doubt) otherwise. There also would have been a real world which Descartes perceived, as the Meditations progressively reveal to be the case, whatever deceptions the evil demon practised on Descartes, Med. I/ start II. (I endorse the Meditations for the sake of argument.)The fact that Descartes does not know (or cannot be sure) that he is not being deceived does not alter the fact, vindicated by the remainder of the Meditations, that he is not in fact being deceived. In these conditions, there precisely are the right sorts of connection with features of the world outside themselves, connections required for their thoughts to represent what they seem to represent. Descartes' situation, even with the evil demon doing his worst, does not replicate that of the brain in a vat.
_____________________________________________________________________________
Reading
G. Harman, Thought, NY : Princeton, 1973.
Hilary Putnam, Reason, Truth, and History (Cambridge: Cambridge University Press) 1981.
John Heil, Are We Brains in a Vat?', Canadian Journal of Philosophy, Vol. 17, No. 2 (Jun., 1987), pp. 427-436.
