[FOM] The Lucas-Penrose Thesis

laureano luna laureanoluna at yahoo.es
Fri Sep 29 14:29:03 EDT 2006

On 29 Sep 2006 Panu Raatikainen wrote:

>And what on earth guarantees that a human mind is
>consistent? As I 
>even many eminent logicians have believed in
>inconsistent theories.

I think it is crucial to distinguish the factual
behavior of humans (which is largely inconsistent as
anyone knows) from the logical and mathematical
ability of humans that is not a matter of fact but an
ideal legality; if we mix them up, we are forced to
conclude that the capability we are using to conclude
that we are forced to conclude that ... ... is

I quite agree with Hartley Slater about the ultimate
root of the insurmountable difference between machines
and minds: minds have a semantical dimension. It is a
way of arguing I think Searle initiated. However, in
the well known argument of Searle there is something
missing: Searle was not able to show that the
semantical dimension of mind has any functional role
in cognitive activity. I believe that the
Gaifman-Goldstein-Hodges tokenist tradition provides a
way to fill up this gap. Consider the Strengthened

(1) (1) expresses no true proposition

After realizing that (1) has no truth value, I state:

(2) (1) expresses no true proposition

and I can take (2) as a different object from (1) only
because I notice that (1) and (2) do not express
equivalent thoughts (indeed (1) expresses no thought).

The reasoning above requires reference to semantical
features, since the whole point is that two
syntactically identical sentences have different
semantical values; and I believe there is no possible
disambiguation of all linguistic cases of this kind.

I'll propose you one of the reasons why I'm inclined
to believe I am not a machine. It is not the
Lucas-Penrose argument but it is related in some way.
Let’s call ‘G’ the following sentence-token:

'Laureano Luna Cabañero does not believe G expresses a
true proposition'

I know I am Laureano Luna Cabañero.

I know that if I believed G expresses a true
proposition, G would be false.

This is why I definitely do not believe G expresses a
true proposition.

Since I know that I don’t believe G expresses a true
proposition and that I am Laureano Luna Cabañero, I
know the following:

'Laureano Luna Cabañero does not believe G expresses a
true proposition'

Let’s call ‘G*’ that sentence-token. 

If G expressed a proposition, it would say the same as
G* says, and then I would believe G.

Since I do not believe G, G expresses no proposition.

But if I were a machine, G would always express a
proposition because it would be referring to the
behavior of a machine and any question about the
behavior of a purely mechanical device is always a
well defined state of affairs.

So, I’m not a machine.

Best regards,

Laureano Luna Cabañero

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