Computation is the implementation of a symbol system. A symbol system
involves the manipulation of a set of arbitrary symbols and
symbol-strings by an explicit set of rules (also arbitrary symbols and
strings) on the bases of their shape alone (i.e. by syntax not
semantics). Although the manipulation is based merely on the shape of
the symbols, the system and its symbols must be systematically
interpretable as having meaning.
For a number of reasons cognition has been thought by many to be
computation (e.g. Fodor, 1980; Pylyshyn, 1973, 1984); computation is
all powerful, it is implimentation-independent and its use led to some
successes in modeling human behavioural capacity. However, it was its
main strength that became its down fall.
Computation is all powerful, both formally and physically (The
Church/Turing Thesis). Any attempt to define computation will always be
and has to be the same. This also goes for any particular aspects of it
(e.g. 1+1=2). In a physical sense it is all powerful as any physical
system can be simulated using computation (from the movement of atoms
to the movement of galaxies). If cognition captures everything you can
do formally and physically then why can't it capture what it is that
minds are. The argument is extremely persuasive, especially as the way
computation works seems to be the way certain aspects of our abilities
work at an introspective level, for example, our logical reasoning
capacities.
The aim of psychology (although not everyone would agree) is to explain
how humans do what they do. In a hundred years of the subject there had
not really been any progress until computational systems were made that
could generate behaviour that was like ours. Computationalism, under
the heading of Artificial Intelligence (AI), provided systems that
could replicate our behavioural capacities. Particular successes
included linguistic skills such as answering questions on text, chess
playing, and reasoning skills.
Perhaps the strongest evidence that cognition was computation was the
fact that computation is implimentation-independent. This means that
the output of a symbol system is not dependent on the physical system
it is being run on. In computers, the symbol system is the software and
the physical sysem is the hardware. The same software could be run on
different hardware and the processing and output would be unchanged.
The attraction of this aspect of computation to mind theorists was that
it seemed to give some answer to the mind-body problem which had
escaped philosophers and psychoilogists since at least Descartes' time.
That is, how is it that a causal body explanation could be reached for
the seemingly irreducable property of the mind - consciousness?
Symbolists said this was not a problem as there is no connection
between the brain (the hardware) and the mind (the software) as in any
computatonal sysem the software is independent of the hardware. This
also led to the proposal that if the 'mind program' could be
implimented that produced behaviour indistinguishable from ours on
another hardware sytem (i.e. a computer) it too would be conscious.
Paradoxically, this main benefit of computation became its downfall
enabling it to be shown that a purely symbolic system could not have a
mind. Searle (1980) used the fact that computational systems are by
definition implementation-independent to demonstate this shortfall by
becoming the implementation himself of a hypothetical symbol system
that produced behaviour indistinguishable from ours . His thought
experiment involved a symbol system that could fool a Chinese penpal
into thinking it could understand Chinese (i.e. pass the Turing Test
(Turing,1964) in Chinese). He said that the computer would not
understand Chinese (and thus have a mind) as Searle himself could
implement the symbol system that provided the Chinese text output
without understanding Chinese. If the computer could understand Chinese
then it would be due to the hardware and not the software as proponents
of the symbolic view of mind had claimed.
There is a version of the Turing Test that is immune to the Chinese
Room Argument (the Total Turing Test; Harnad, 1989) in which the
symbols are grounded in the world and do not depend on the
understanding of the observer for their meaning. Cognitive Psychology
is left to attempt to discover how such a system might work while still
retaining the benefits symbolic sysems have in modelling human
behaviour.
This archive was generated by hypermail 2b30 : Tue Feb 13 2001 - 16:23:57 GMT