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

“any large computation should be split up and implemented as a collection of small
sub-parts that are as nearly independent of one another as the overall task allows”
(p. 485). Failure to devise a functional component or process according to the principle
of modular design typically means,
that the process as a whole becomes extremely difficult to debug or to improve,
whether by a human designer or in the course of natural evolution, because a small
change to improve one part has to be accompanied by many simultaneous compensating
changes elsewhere. (Marr, 1976, p. 485)
Digital computers were explicitly designed according to the principle of modular
design, which von Neumann (1958) called “the principle of only one organ for
each basic operation” (p. 13). Not only was this good engineering practice, but von
Neumann also argued that this principle distinguished digital computers from their
analog ancestors such as the differential analyzer (Bush, 1931).
The principle of modular design is also reflected in the architecture of the universal
Turing machine. The central control organ of this device is its machine table
(see Figure 3-8), which is separate and independent from the other elements of the
device, such as the mechanisms for reading and writing the tape, the machine state,
and so on. Recall that the machine table is a set of instructions; each instruction
is associated with a specific input symbol and a particular machine state. When
a Turing machine in physical state x reads symbol y from the tape, it proceeds to
execute the instruction at coordinates (x, y) in its machine table.
Importantly, completely decentralized control results in a Turing machine
when von Neumann’s (1958) principle of only one organ for each basic operation
is taken to the extreme. Rather than taking the entire machine table as a central
control organ, one could plausibly design an uber-modular system in which each
instruction was associated with its own organ. For example, one could replace the
machine table with a production system in which each production was responsible
for one of the machine table’s entries. The conditions for each production would
be a particular machine state and a particular input symbol, and the production’s
action would be the required manipulation of the ticker tape. In this case, the production
system version of the Turing machine would behave identically to the original
version. However, it would no longer have a centralized control organ.
In short, central control is not a necessary characteristic of classical information
processing, and therefore does not distinguish between classical and embodied
theories. Another way of making this point is to remember the Chapter 3 observation
that production systems are prototypical examples of classical architectures
(Anderson et al., 2004; Newell, 1973), but they, like many embodied models
(Dawson, Dupuis, & Wilson, 2010; Holland & Melhuish, 1999; Susi & Ziemke, 2001;
Theraulaz & Bonabeau, 1999), are controlled stigmergically. “Traditional production
system control is internally stigmergic, because the contents of working memory
332 Chapter 7