Fetzer assures us
elsewhere
that people do advance the bald thesis that
persons are Turing machines. They certainly do. In fact,
though there may be a need to tinker
with this proposition,
it captures a large part of computationalism in *charitable* fashion.
One reason this is so is because it doesn't follow from the fact
that *x* is a TM that some process that *x* undergoes must be
formalizable as (say) Turing computation of some effectively computable
function.
I would respectfully suggest that Cummins and Schwarz [8]
-- who, as Fetzer notes in section 7 of his [11],
propound the chain that
**computing** is the execution of **functions**,
which entails the execution of **algorithms**, which in turn
involves **disciplined step satisfaction** -- take note of this fact.
Specific examples are easy to come by:
a composite TM could include a ``sub" TM that
nondeterministically attempts to find composite numbers

In general,
it seems to me that computability theory provides ``off the shelf"
objects and concepts sufficient to formalize
*all* the distinctions Fetzer finds computationalists making.
For another example,
consider ``asymmetrical decision procedures," to which
the computationalist Johnson-Laird
[16],
as Fetzer notes elsewhere [12], appeals.
Fetzer cites as an example of such a procedure
the classic one for establishing that the set of all proofs in first-order
logic is enumerable. It's of course well-known
that there is a TM that performs the
work in question; the problem here is known to be in the
Arithmetic Hierarchy ( and formalize
that
which can be solved by any of the techniques open to
standard computationalism). In general, then,
my advice to computationalists who come
under Fetzer's scrutiny is to drop all reference to ersatz computation
and embrace the rich but certified offerings of computability theory.

Fetzer himself believes that at least the spirit of my recommendation is (imprudently) affirmed by computationalists, for he says:

[T]he computational conception appears to have arisen from the almost irresistible temptation to appeal to a domain about which a great deal is known (formal systems and computability theory) as a repository of answers to questions in a domain about which very little is known (the nature of mentality and cognition) (p. 25, [12]).

But I offer analogous advice for
some of those intent on *razing*
computationalism.
For example, Penrose's objection (which
Fetzer reports and discusses in [12]) that thinking
can't be computation because computation is deterministic is a weak
one in light of the fact that TMs can be allowed to move from state
to state in nondeterministic and probabilistic fashion. Such machines
don't have state transition *functions*; transitions are simply
*n*-tuples from an appropriate relation on states and symbols.

Fetzer does consider an approximation of the move of simply identifying persons with TMs, as can be seen by considering what he has to say about dreams. Fetzer says:

Many kinds of thought processes of human beings as thinking things fail to satisfy [the conditions that determined computation].Dreamsanddaydreams, for example, fail to satisfy them because they have definite starting point and no definite stopping point: they begin and they end, but they have no given premises or conclusions" (sect. 8, para. 1).

But he also tells us that

A computationalist could insist that dreams and daydreams, properly understood, really are instances of disciplined step satisfaction (or of the execution of complete or partial functions) by observing that, even in the case of dreams or daydreams, a system simply shifts from state at time to state at time in some specific sequence. ([12], p. 24)

Immediately after considering this version of the heart of computationalism, Fetzer says that, with respect to the system in question,

One could insist that any system that passes through those states is computing some particular function -- the functionfthat takes input at and returns output at . This defense, however, trivializes computationalism by ignoring the difference between causal processes and computational procedures. ([12], p. 24; see also [11], sect. 9, para. 3)

It is possible that I could be persuaded here, but my suspicion is that a clever computationalist will insist that the only outright function that needs to figure in a description of an AI is one mapping percepts to behavior. So for example a Turing machine that starts with one on its tape and simply copies that over and over again ``forever" is doing enough, in principle, to count as an artificial agent. (For a marvelous introduction to AI based on this percept behavior conception, see [18].) At any rate, it isn't Fetzer's intention, nor is it mine, to set out computationalism in glorious detail. All that is needed at present is a rough-and-ready characterization of computationalism, and here's what I suggest:

**Computationalism** consists of the following four propositions.

**CTT**- A function
*f*is effectively computable if and only if*f*is Turing-computable. **P=aTM**- Persons are Turing machines.
**TT**- The Turing Test is valid.
**P-BUILD**- Computationalists will succeed in building persons.
**TT-BUILD**- Computationalists will succeed in building Turing Test-passing
artifacts. (This proposition is presumably entailed by its predecessor.)

Tue May 21 00:31:50 EDT 1996