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

Comparisons between different devices are further complicated by introducing
the notion of an architecture (Brooks, 1962). In computer science, the term
architecture was originally used by Frederick P. Brooks Jr., a pioneering force in
the creation of IBM’s early computers. As digital computers evolved, computer
designers faced changing constraints imposed by new hardware technologies. This
is because new technologies defined anew the basic information processing properties
of a computer, which in turn determined what computers could and could not
do. A computer’s architecture is its set of basic information processing properties
(Blaauw & Brooks, 1997, p. 3): “The architecture of a computer system we define
as the minimal set of properties that determine what programs will run and what
results they will produce.”
The two different versions of Shannon’s (1938) selective circuit were both based
on the same architecture: the architecture’s primitives (its basic components) were
parallel and serial combinations of pairs of switches. However, other sets of primitives
could be used.
An alternative architecture could use a larger number of what Shannon (1938)
called special types of relays or switches. For instance, we could take each of the
16 logical functions listed in Table 2-2 and build a special device for each. Each
device would take two currents as input, and would convert them into an appropriate
output current. For example, the XOR device would only deliver a current if
only one of its input lines was active; it would not deliver a current if both its input
lines were either active or inactive—behaving exactly as it is defined in Table 2-2. It
is easy to imagine building some switching circuit that used all of these logic gates
as primitive devices; we could call this imaginary device “circuit x.”
The reason that the notion of architecture complicates (or enriches!) the comparison
of devices is that the same circuit can be created from different primitive
components. Let us define one additional logic gate, the NOT gate, which does not
appear in Table 2-2 because it has only one input signal. The NOT gate reverses or
inverts the signal that is sent into it. If a current is sent into a NOT gate, then the
NOT gate does not output a current. If a current is not sent into a NOT gate, then
the gate outputs a current. The first NOT gate—the first electromechanical relay—
was invented by American physicist Joseph Henry in 1835. In a class demonstration,
Henry used an input signal to turn off an electromagnet from a distance, startling
his class when the large load lifted by the magnet crashed to the floor (Moyer, 1997).
The NOT gate is important, because it can be used to create any of the Table 2-2
operations when combined with two other operators that are part of that table: AND,
which McCulloch represented as pq, and OR, which McCulloch represented as p q.
To review, if the only special relays available are NOT, A,ND and OR, then one can use
these three primitive logic blocks to create any of the other logical operations that are
Multiple Levels of Investigation 33