Present day biological knowledge allows us to say that a living system considered as a unity in the physical space, that is, as an entity topologically and operationally separable from the physical background, is defined by an organization that consists of a network of processes of production and transformation of components, molecular and otherwise, that through their interactions: a) recursively generate the same network of processes of production of components that generated them; and b) constitute the system as a physical unity by determining its boundaries in the physical space. This organization I call the autopoietic system. Due to this organization a living system is an autonomous unity, self-assertive in its dynamic capacity to wothstand deformation under continuous turnover of matter while remaining invariant in its organization.

Since a living system is defined as a unity by its autopoietic organization, all the transformations that it may undergo without losing its identity are transformations in which its organization remains invariant: an autopoietic system is a homeostatic system that has its own organization as the essential variable that it maintains constant through its operation. Therefore, all the unitary phenomena of an autopoietic system are constitutively subordinated to the maintenance of its autopoiesis. (CS 460)