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

out at any given time (Newell, 1980; Simon, 1969). To detail the architecture is to
specify “what operations are primitive, how memory is organized and accessed, what
sequences are allowed, what limitations exist on the passing of arguments and on the
capacities of various buffers, and so on” (Pylyshyn, 1984, p. 92).
What is the relationship between an algorithm and its architecture? In general,
the architecture provides the programming language in which an algorithm is written.
“Specifying the functional architecture of a system is like providing a manual
that defines some programming language. Indeed, defining a programming language
is equivalent to specifying the functional architecture of a virtual machine”
(Pylyshyn, 1984, p. 92).
This means that algorithms and architectures share many properties. Foremost
of these is that they are both described as operations, behaviours, or functions,
and not in terms of physical makeup. An algorithm is a set of functions that work
together to accomplish a task; an architectural component is one of the simplest
functions—a primitive operation—from which algorithms are composed. In order
to escape Ryle’s regress, one does not have to replace an architectural function with
its physical account. Instead, one simply has to be sure that such a replacement is
available if one wanted to explain how the architectural component works. It is no
accident that Pylyshyn (1984) uses the phrase functional architecture in the quote
given above.
Why do we insist that the architecture is functional? Why don’t we appeal
directly to the physical mechanisms that bring an architecture into being? Both of
these questions are answered by recognizing that multiple physical realizations are
possible for any functional architecture. For instance, simple logic gates are clearly
the functional architecture of modern computers. But we saw earlier that functionally
equivalent versions of these gates could be built out of wires and switches,
vacuum tubes, semiconductors, or hydraulic valves.
To exit Ryle’s regress, we have to discharge an algorithm’s homunculi. We can
do this by identifying the algorithm’s programming language—by saying what its
architecture is. Importantly, this does not require us to say how, or from what physical
stuff, the architecture is made! “Whether you build a computer out of transistors,
hydraulic valves, or a chemistry set, the principles on which it operates are
much the same” (Hillis, 1998, p. 10).
2.11 Implementing Architectures
At the computational level, one uses a formal vocabulary to provide a rigorous
description of input-output mappings. At the algorithmic level, a procedural or
behavioural vocabulary is employed to describe the algorithm being used to calculate
a particular input-output mapping. The functional architecture plays a special
48 Chapter 2