A system is less a thing than a pattern. It is a pattern of events, its existence and character deriving less from the nature of its components than from their organization. As such it consists of a dynamic flow of interactions. It is “non-summative” and irreducible; that is, the character of a system as a pattern of organization is altered with the addition, subtraction or modification of any piece. Hence it is more than the sum of its parts. This “more” is not something extra, like a vitalist principle or an élan vital, but a new level of operation which the interdependence of its parts permits. It is lost from view when a system’s composite units are investigated independently.
Joanna Macy, Mutual Causality in Buddhism and General Systems Theory (1991), 72