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

apidly switch currents on and off and to amplify weak signals (Reid, 2001). Vacuum
tubes were replaced by transistors built from semiconducting substances such as
silicon. Ultimately, transistors were miniaturized to the point that millions could be
etched into a single silicon chip.
One might suggest that the examples listed above are not as physically different
as intended, because all are electrical in nature. But relays can be implemented
in many nonelectrical ways as well. For example, nanotechnology researchers
are exploring various molecular ways in which to create logic gates (Collier et al.,
1999; Okamoto, Tanaka, & Saito, 2004). Similarly, Hillis (1998) described in detail
a hydraulic relay, in which the source and drain involve a high-pressure water line
and a weaker input flow controls a valve. He pointed out that his hydraulic relay is
functionally identical to a transistor, and that it could therefore be used as the basic
building block for a completely hydraulic computer. “For most purposes, we can
forget about technology [physical realization]. This is wonderful, because it means
that almost everything that we say about computers will be true even when transistors
and silicon chips become obsolete” (p. 19).
Multiple realization is a key concept in cognitive science, particularly in classical
cognitive science, which is the topic of Chapter 3. Multiple realization is in
essence an argument that while an architectural account of a system is critical,
it really doesn’t matter what physical substrate is responsible for bringing the
architecture into being. Methodologically this is important, because it means that
computer simulation is a viable tool in cognitive science. If the physical substrate
doesn’t matter, then it is reasonable to emulate the brain-based architecture of
human cognition using completely different hardware—the silicon chips of the
digital computer.
Theoretically, multiple realization is also important because it raises the possibility
that non-biological systems could be intelligent and conscious. In a famous
thought experiment (Pylyshyn, 1980), each neuron in a brain is replaced with a silicon
chip that is functionally equivalent to the replaced neuron. Does the person
experience any changes in consciousness because of this change in hardware? The
logical implication of multiple realization is that no change should be experienced.
Indeed, the assumption that intelligence results from purely biological or neurological
processes in the human brain may simply be a dogmatic attempt to make
humans special when compared to lower animals or machines (Wiener, 1964, p. 31):
“Operative images, which perform the functions of their original, may or may not
bear a pictorial likeness to it. Whether they do or not, they may replace the original
in its action, and this is a much deeper similarity.”
Multiple Levels of Investigation 37