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

A production system is a general-purpose symbol manipulator (Anderson, 1983;
Newell, 1973; Newell & Simon, 1972). Like other physical symbol systems, production
systems exhibit a marked distinction between symbolic expressions and the
rules for manipulating them. They include a working memory that is used to store
one or more symbolic structures, where a symbolic structure is an expression that
is created by combining a set of atomic symbols. In some production systems (e.g.,
Anderson, 1983) a long-term memory, which also stores expressions, is present as
well. The working memory of a production system is analogous to the ticker tape
of a Turing machine or to the random access memory of a von Neumann computer.
The process component of a production system is a finite set of symbol-manipulating
rules that are called productions. Each production is a single rule that pairs
a triggering condition with a resulting action. A production works by scanning the
expressions in working memory for a pattern that matches its condition. If such a
match is found, then the production takes control of the memory and performs its
action. A production’s action is some sort of symbol manipulation—adding, deleting,
copying, or moving symbols or expressions in the working memory.
A typical production system is a parallel processor in the sense that all of its
productions search working memory simultaneously for their triggering patterns.
However, it is a serial processor—like a Turing machine or a digital computer—
when actions are performed to manipulate the expressions in working memory.
This is because in most production systems only one production is allowed to operate
on memory at any given time. That is, when one production finds its triggering
condition, it takes control for a moment, disabling all of the other productions. The
controlling production manipulates the symbols in memory, and then releases its
control, which causes the parallel scan of working memory to recommence.
We have briefly described two characteristics, structure and process, that make
production systems examples of physical symbol systems. The third characteristic,
control, reveals some additional interesting properties of production systems.
On the one hand, stigmergy is used to control a production system, that is,
to choose which production acts at any given time. Stigmergic control occurs
when different agents (in this case, productions) do not directly communicate
with each other, but conduct indirect communication by modifying a shared
environment (Theraulaz & Bonabeau, 1999). Stigmergy has been used to explain
how a colony of social insects might coordinate their actions to create a nest
(Downing & Jeanne, 1988; Karsai, 1999). The changing structure of the nest elicits
different nest-building behaviours; the nest itself controls its own construction.
When one insect adds a new piece to the nest, this will change the later behaviour of
other insects without any direct communication occurring.
Production system control is stigmergic if the working memory is viewed
as being analogous to the insect nest. The current state of the memory causes a
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