International Encyclopedia of Systems and Cybernetics

"A unit of a system that in combination with other units functions to combine, separate, or compare the inputs to produce outputs" (K. BERRIEN, 1968, p.32).

"An integral part of a system, selected on the basis of its potential to carry out functions required for the achievement of the system's goal" (B. BANATHY, 1973, p.85).

The BERRIEN's definition should be compared with J. MILLER's subsystems, mainly, but not exclusively, the converter, the producer and the associator. The outputs may be intra-systemic, for the use of other subsystems. It also seems that components may carry out other tasks in a system. BERRIEN emphasizes that a critical proximity is necessary for components to interact. He adds that "Once embedded in a system, the component to some degree becomes dependent upon the total system for its existence. The greater its specialization, the greater is the dependence of the component on the total system" (p.84).

The distinction of a component is partly a matter of viewpoint: A component can generally be viewed as a whole system when considered at a lower hierarchic level. A cell is a complex system in its own right, but in a living being it acts as a component, because it has entered in a net of relationships. In the same sense, a (complex) atom is a component in a molecule, and an individual, itself biologically a complex system, is a component in a group or organization.

Generally, components are integrated in some more or less permanent structures. However, in some cases they may be exchanged between two or more subsystems or even circulate more or less freely within the system. Nonetheless, the individual properties of the components become restricted as an effect of their integration within a specific system, while they may become associated with other components and produce some collective behavior which none of them could produce individually. This is why systemists say that a system is more (and less) than the sum of its components.

In another aspect, the components of a system are frequently incompatible with other systems. This is the root of immunological rejection and probably, of feuds between nations, cultures, religions and races.

The reproduction process of self compatible components by the system is a basic part of the autopoietic process.

In relation to higher levels of complexity, components of lower levels frequently belong to a quite limited number of types. There are for example less than 100 basic types of atoms; only 4 nucleotids in DNA chains; and 20 amino-acids for the makeup of proteins. More examples are the basic binary units in computers or the 20-plus letters in most languages alphabets. As observed by H. SIMON, a small number of basic components at any level produces simplicity through redundancy. (see also: C. BRESCH, 1977).