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

By the late 1950s, it became conventional to load the program—then known as
the “short code” (von Neumann, 1958)—into memory. This is called memory-stored
control; the first modern computer to use this type of control was Manchester’s
“Baby” (Lavington, 1980). In Chapter 2 we saw an example of this type of control in
the universal Turing machine, whose ticker tape memory holds both the data to be
manipulated and the description of a special-purpose Turing machine that will do
the manipulating. The universal Turing machine uses the description to permit it to
pretend to be the specific machine that is defined on its tape (Hodges, 1983).
In a physical symbol system that employs memory-stored control, internal
characteristics will vary over time. However, the time scale of these changes will not
be uniform (Newell, 1990). The data that is stored in memory will likely be changed
rapidly. However, some stored information—in particular, the short code, or what
cognitive scientists would call the virtual machine (Pylyshyn, 1984, 1991), that controls
processing would be expected to be more persistent. Memory-stored control in
turn chooses which architectural operation to invoke at any given time. In a digital
computer, the architecture would not be expected to vary over time at all because it
is fixed, that is, literally built into the computing device.
The different characteristics of a physical symbol system provide a direct link
back to the multiple levels of investigation that were the topic of Chapter 2. When
such a device operates, it is either computing some function or solving some information
processing problem. Describing this aspect of the system is the role of a
computational analysis. The computation being carried out is controlled by an
algorithm: the program stored in memory. Accounting for this aspect of the system
is the aim of an algorithmic analysis. Ultimately, a stored program results in the
device executing a primitive operation on a symbolic expression stored in memory.
Identifying the primitive processes and symbols is the domain of an architectural
analysis. Because the device is a physical symbol system, primitive processes and
symbols must be physically realized. Detailing the physical nature of these components
is the goal of an implementational analysis.
The invention of the digital computer was necessary for the advent of classical
cognitive science. First, computers are general symbol manipulators. Their
existence demonstrated that finite devices could generate an infinite potential
of symbolic behaviour, and thus supported a materialist alternative to Cartesian
dualism. Second, the characteristics of computers, and of the abstract theories of
computation that led to their development, in turn resulted in the general notion
of physical symbol system, and the multiple levels of investigation that such systems
require.
The final link in the chain connecting computers to classical cognitive science
is the logicist assumption that cognition is a rule-governed symbol manipulation of
the sort that a physical symbol system is designed to carry out. This produces the
Elements of Classical Cognitive Science 77