A theory that human society can be studied as a system broken down into many interacting subsystems, or parts. It is the premise that any organization may be looked at to discover how its parts are related and how changes in either parts or their relationships produce changes in the overall system. In archaeological terms, the system might be the whole of a society's culture, or some part of it such as the economy or even a single settlement. Systems can be regarded as either open or closed; the latter have no input of energy or matter from the outside, tend to reach a state of stable equilibrium in which small changes can be offset, and eventually stagnate and disintegrate, while open systems have an input of energy from the outside, reach a state of unstable equilibrium in which any small change can produce significant transformations in the system as a whole, and are characterized by growth and change. The process by which a system tends to maintain equilibrium in the face of changed surroundings is termed homeostasis, while morphogenesis is the process by which the structure is changed or elaborated.