Haugeland offers a theory of systems along the following lines:
A component is a relatively independent and self-contained
portion of a system in the sense that it relevantly interacts with
other components only through interfaces between them (and contains
no internal interfaces at the same level). An interface is a
point of interactive “contact” between components such that the
relevant interactions are well-defined, reliable, and relatively
simple. A system is a relatively independent and
self-contained composite of components interacting at interfaces
(Haugeland, 1998, p. 213.)
Where Haugeland’s theory of system might work well for some
electronic systems, it fares much less well for some biological
systems. There are biological systems that are individuated, in
part, by their structure and the kinds of processes they carry out,
rather than exclusively by their interfaces. As an example, consider
the human muscular system.
The set of muscles in the human body constitute the muscular system.
In general, the muscles in the muscular system do not interface with
each other; hence do not constitute a system in Haugeland’s sense.
Even antagonistic muscles, such as the lateral and medial rectus
muscles of the eye, which move the eye left and right, do not connect
to each other. But, even if one were to say that antagonistic
muscles have a kind of indirect interface, there are other
combinations of muscles that do not stand in such antagonistic
relations. The lateral and medial rectus muscles of the eye do not,
for example, interface in any natural way with the gastrocnemius
muscles of the calves. What appears to unify the muscles of the body
into a system appears to be a commonality of function and a
commonality of underlying mechanism.