On Constructing a Reality - Understanding Understanding: Essays on Cybernetics and Cognition - page 217

Hence

cognition Æ computations of

In summary, I propose to interpret cognitive processes as never-
ending recursive processes of computation, and I hope that in the follow-
ing tour de force of neurophysiology I can make this interpretation

transparent.

Neurophysiology

Evolution
In order that the principle of recursive computation be fully appreciated as
being the underlying principle of all cognitive processes—even of life itself,
as one of the most advanced thinkers in biology assures me 5 —it may be
instructive to go back for a moment to the most elementary—or as evolu-
tionists would say, to very “early”—manifestations of this principle. These
are the “independent effectors,” or independent sensorimotor units, found
in protozoa and metazoa distributed over the surface of these animals
(Fig. 7). The triangular portion of this unit, protruding with its tip from the
surface, is the sensory part; the onion-shaped portion, the contractile motor
part. A change in the chemical concentration of an agent in the immediate
vicinity of the sensing tip, and “perceptible” by it, causes an instantaneous
contraction of this unit. The resulting displacement of this or any other unit
by change of shape of the animal or its location may, in turn, produce per-
ceptible changes in the agent's concentration in the vicinity of these units,
which, in turn, will cause their instantaneous contraction, and so on. Thus
we have the recursion

change of sensation Æ change of shape

Separation of the sites of sensation and action appears to have been the
next evolutionary step (Fig. 8). The sensory and motor organs are now con-
nected by thin filaments, the “axons” (in essence degenerated muscle fibers
having lost their contractility), which transmit the sensor's perturbations to
its effector, thus giving rise to the concept of a “signal”: See something here,
act accordingly there.
The crucial step, however, in the evolution of the complex organization
of the mammalian central nervous system (CNS) appears to be the appear-