ITT: Talcott Parsons

Let us now turn to a more detailed discussion of our conception of a social system.
First, the concept of interpenetration implies that, however important logical closure
may be as a theoretical ideal, empirically social systems are conceived as open
systems, engaged in complicated processes of interchange with environing systems.
The environing systems include, in this case, cultural and personality systems, the
behavioral and other subsystems of the organism, and, through the organism, the
physical environment. The same logic applies internally to social systems, conceived
as differentiated and segmented into a plurality of subsystems, each of which must
be treated analytically as an open system interchanging with environing subsystems
of the larger system.
The concept of an open system interchanging with environing systems also
implies boundaries and their maintenance. When a set of interdependent phenomena
shows sufficiently definite patterning and stability over time, then we can say
that it has a "structure" and that it is fruitful to treat it as a "system." A boundary
means simply that a theoretically and empirically significant difference between
structures and processes internal to the system and those external to it exists and
tends to be maintained. In so far as boundaries in this sense do not exist, it is not
possible to identify a set of interdependent phenomena as a system; it is merged in
some other, more extensive system. It is thus important to distinguish a set of phenomena
not meant to constitute a system in the theoretically relevant sense - e.g.,
a certain type of statistical sample of a population - from a true system.
Structural and Functional Modes of Analysis. Besides identifying a system in
terms of its patterns and boundaries, a social system can and should be analyzed in
terms of three logically independent - i.e., cross-cutting - but also interdependent,
bases or axes of variability, or as they may be called, bases of selective abstraction.
The first of these is best defined in relation to the distinction between "structural"
and "functional" references for analysis. However relative these two concepts
may be, the distinction between them is highly important. The concept of
structure focuses on those elements of the patterning of the system which may be regarded as independent of the lower-amplitude and shorter time-range fluctuations
in the relation of the system to its external situation. It thus designates the features
of the system which can, in certain strategic respects, be treated as constants over
certain ranges of variation in the behavior of other significant elements of the theoretical
problem.
Thus, in a broad sense, the American Constitution has remained a stable reference
point over a period of more than a century and a half. During this time, of
course, the structure of American society has changed very greatly in certain
respects; there have been changes in legal terms, through legislation, through legal
interpretations, and through more informal processes. But the federal state, the division
between legislative and executive branches of government, the independent
judiciary, the separation of church and state, the basic rights of personal liberty, of
assembly, and of property, and a variety of other features have for most purposes
remained constant.
The functional reference, on the other hand, diverges from the structural in the
"dynamic" direction. Its primary theoretical significance is integrative; functional
considerations relate to the problem of mediation between two fundamental sets of
exigencies: those imposed by the relative constancy or "givenness" of a structure,
and those imposed by the givenness of the environing situation external to the
system. Since only in a theoretically limiting case can these two be assumed to stand
in a constant relation to each other, there will necessarily exist a system of dynamic
processes and mechanisms