Humberto Maturana and Francisco Varela developed the idea of autopoiesis (meaning self-making) as a general definition of both living and cognitive systems. One of their descriptions of autopoiesis is through the idea of an autopoietic machine:
Autopoietic machines are homeostatic machines. Their peculiarity, however, does not lie in this but in the fundamental variable which they maintain constant. An autopoietic machine is a machine organized (defined as a unity) as a network of processes of production (transformation and destruction) of components that produces the components which: (i) through their interactions and transformations continuously regenerate and realize the network of processes (relations) that produced them; and (ii) constitute it (the machine) as a concrete unity in the space in which they (the components) exist by specifying the topological domain of its realization as such a network. It follows that an autopoietic machine continuously generates and specifies its own organization through its operation as a system of production of its own components, and does this in an endless turnover of components under conditions of continuous perturbations and compensation of perturbations. Therefore, an autopoietic machine is an homeostatic (or rather a relations-static) system which has its own organization (defining network of relations) as the fundamental variable which it maintains constant. This is to be clearly understood.
Humberto Maturana and Francisco Varela, Autopoiesis and Cognition, 1972
With this definition they encompass the ideas that in general, living systems maintain a certain stability, that they are networks, they have dynamic metabolism and that they have a circular causality (the system is organised to as to maintain the system's organisation). They reinforced this idea with the concepts of organisation and structure.
In 'The Web of Life', Fritjof Capra (author of the seminal art/science work, 'The Tao of Physics') clarifies and develops these ideas further. As he explains:
A New Synthesis
We can now return to the central question of this book - what is life? My thesis has been that a theory of living systems consistent with the philosophical framework of deep ecology, including an appropriate mathematical language, and implying a non-mechanistic, post-Cartesian understanding of life, is now emerging.
The emergence and refinement of the concept of 'pattern of organization' has been a crucial element in the development of this new way of thinking. From Pythagoras to Aristotle, to Goethe, and to the organismic biologists there is a continuous intellectual tradition that struggles with the understanding of pattern, realizing that it is crucial to the understanding of living form. Alexander Bogdanov was the first to attempt the integration of the concepts of organization, pattern, and complexity into a coherent systems theory. The cyberneticists focused on patterns of communication and control - in particular on the patterns of circular causality underlying the feedback concept - and in doing so were the first to clearly distinguish the pattern of organization of a system from its physical structure.
The missing 'pieces of the puzzle' were identified and analysed over the past twenty years - the concept of self-organization and the new mathematics of complexity. Again, the notion of pattern has been central to both of these developments. The concept of self-organization originated in the recognition of the network as the general pattern of life, which was subsequently refined by Maturana and Varela in their concept of autopoiesis. The new mathematics of complexity is essentially a mathematics of visual patterns - strange attractors, phase portraits, fractals, etc.- which are analysed within the framework of topology pioneered by Poincaré.
The understanding of pattern, then, will be of crucial importance to the scientific understanding of life. However, for a full understanding of a living system, the understanding of its pattern of organization, although critically important, is not enough. We also need to understand the system's structure. Indeed, we have seen that the study of structure has been the principal approach in Western science and philosophy and as such has again and again eclipsed the study of pattern.
I have come to believe that the key to a comprehensive theory of living systems lies in the synthesis of those two approaches - the study of pattern (or form, order, quality) and the study of structure (or substance, matter, quantity). I shall follow Humberto Maturana and Francisco Varela in their definitions of those two key criteria of a living system - its pattern of organization and its structure.
The pattern of organization of any system, living or nonliving, is the configuration of relationships among the system's components that determines the system's essential characteristics. In other words, certain relationships must be present for something to be recognized as - say - a chair, a bicycle, or a tree. That configuration of relationships that gives a system its essential characteristics is what we mean by its pattern of organization.
The structure of a system is the physical embodiment of its pattern of organization. Whereas the description of the pattern of organization involves an abstract mapping of relationships, the description of the structure involves describing the system's actual physical components - their shapes, chemical compositions, etc.
from The Web of Life, Fritjof Capra, 1996
He then goes on to consider process as a third element which is the means by which organisation is continually embodied, and so concludes with three key criteria of living systems:
from The Web of Life, Fritjof Capra, 1996