International Encyclopedia of Systems and Cybernetics

"A system made up of a large number of parts, that interact in a non simple way." (H. SIMON, 1965, p.63).

H. SIMON comments this informal definition as follows:" In such systems, the whole is more than the sum of the parts, not in an ultimate, metaphysical sense, but in the important pragmatic sense that, given the properties of the parts and the laws of their interaction, it is not a trivial matter to infer the properties of the whole. In the face of complexity, an in-principle reductionist may be at the same time a pragmatic holist" (p.63).

One should add that the complex system is altogether in some sense less than the sum of its parts, since these always abandon some of their properties, as they become inhibited by some constraints resulting of the global or some particular interactions.

The complex system is thus not merely complicated, as these constraints are generally of various kinds and reciprocally constraining. Complexity is organized and the complex system presents organizational closure.

Of course, from a different viewpoint, complexity is in the eye of the observer, as explained by R. ROSEN: "We are going to define a complex system as one with which we can interact effectively in many different kinds of ways, each requiring a different mode of description" (1977, p.229).

J.L. LEMOIGNE proposes the following nine characteristics each one of which increases the complexity of a system: 1) to be identifiable; 2) to be active; 3) to be regulated; 4) to be informed about its own behavior; 5) to be able to decide its own behavior; 6) to be endowed with memory: 7) to be able to coordinate its behavioral decisions; 8) to be able to imagine or conceive new possible decisions and 9) to be able to finalize itself (1990).