Mind, Body, World- Foundations of Cognitive Science, 2013a

In the Jacquard loom, punched cards control processes that operate on local
components of the “expression” being weaved. The same is true of the physical
symbol systems. Physical symbol systems are finite devices that are capable of producing
an infinite variety of potential behaviour. This is possible because the operations
of a physical symbol system are recursive. However, this explanation is not
complete. In addition, the rules of a physical symbol system are local or componential,
in the sense that they act on local components of an expression, not on the
expression as a whole.
For instance, one definition of a language is the set of all of its grammatical
expressions (Chomsky, 1957). Given this definition, it is logically possible to treat
each expression in the set as an unanalyzed whole to which some operation could
be applied. This is one way to interpret a behaviourist theory of language (Skinner,
1957): each expression in the set is a holistic verbal behaviour whose likelihood of
being produced is a result of reinforcement and stimulus control of the expression
as a whole.
However, physical symbol systems do not treat expressions as unanalyzed
wholes. Instead, the recursive rules of a physical symbol system are sensitive to the
atomic symbols from which expressions are composed. We saw this previously in
the example of context-free grammars that were used to construct the phrase markers
of Figures 3-6 and 3-7. The rules in such grammars do not process whole phrase
markers, but instead operate on the different components (e.g., nodes like S, N, VP)
from which a complete phrase marker is constructed.
The advantage of operating on symbolic components, and not on whole expressions,
is that one can use a sequence of very basic operations—writing, changing,
erasing, or copying a symbol—to create an overall effect of far greater scope than
might be expected. As Henry Ford said, nothing is particularly hard if you divide
it into small jobs. We saw the importance of this in Chapter 2 when we discussed
Leibniz’ mill (Leibniz, 1902), the Chinese room (Searle, 1980), and the discharging
of homunculi (Dennett, 1978). In a materialist account of cognition, thought is produced
by a set of apparently simple, mindless, unintelligent actions—the primitives
that make up the architecture.
The small jobs carried out by a physical symbol system reveal that such a
system has a dual nature (Haugeland, 1985). On the one hand, symbol manipulations
are purely syntactic—they depend upon identifying a symbol’s type, and not
upon semantically interpreting what the symbol stands for. On the other hand, a
physical symbol system’s manipulations are semantic—symbol manipulations preserve
meanings, and can be used to derive new, sensible interpretations.
Interpreted formal tokens lead two lives: syntactical lives, in which they are meaningless
markers, moved according to the rules of some self-contained game; and
Elements of Classical Cognitive Science 79