A holon (Greek: ὅλον, holon neuter form of ὅλος, holos "whole") is something that is simultaneously a whole and a part. The word was coined by Arthur Koestler in his book The Ghost in the Machine (1967, p. 48). Koestler was compelled by two observations in proposing the notion of the holon. The first observation was influenced by Nobel Prize winner Herbert A. Simon's parable of the two watchmakers, wherein Simon concludes that complex systems will evolve from simple systems much more rapidly if there are stable intermediate forms present in that evolutionary process than if they are not present. The second observation was made by Koestler himself in his analysis of hierarchies and stable intermediate forms in both living organisms and social organizations. He concluded that, although it is easy to identify sub-wholes or parts, wholes and parts in an absolute sense do not exist anywhere. Koestler proposed the word holon to describe the hybrid nature of sub-wholes and parts within in vivo systems. From this perspective, holons exist simultaneously as self-contained wholes in relation to their sub-ordinate parts, and dependent parts when considered from the inverse direction.
Koestler also says holons are autonomous, self-reliant units that possess a degree of independence and handle contingencies without asking higher authorities for instructions. These holons are also simultaneously subject to control from one or more of these higher authorities. The first property ensures that holons are stable forms that are able to withstand disturbances, while the latter property signifies that they are intermediate forms, providing a context for the proper functionality for the larger whole.
Finally, Koestler defines a holarchy as a hierarchy of self-regulating holons that function first as autonomous wholes in supra-ordination to their parts, secondly as dependent parts in sub-ordination to controls on higher levels, and thirdly in coordination with their local environment.
General definition 
A holon is a system (or phenomenon) which is an evolving self-organizing dissipative structure, composed of other holons, whose structures exist at a balance point between chaos and order. It is maintained by the throughput of matter–energy and information–entropy connected to other holons and is simultaneously a whole in and itself at the same time being nested within another holon and so is a part of something much larger than itself. Holons range in size from the smallest subatomic particles and strings, all the way up to the multiverse, comprising many universes. Individual humans, their societies and their cultures are intermediate level holons, created by the interaction of forces working upon us both top-down and bottom-up. On a non-physical level, words, ideas, sounds, emotions—everything that can be identified—is simultaneously part of something, and can be viewed as having parts of its own, similar to sign in regard of semiotics. Defined in this way, holons are related to the concept of autopoiesis, especially as it was developed in the application of Stafford Beer to second-order cybernetics and viable system theory, but also Niklas Luhmann in his social systems theory.
Since a holon is embedded in larger wholes, it is influenced by and influences these larger wholes. And since a holon also contains subsystems, or parts, it is similarly influenced by and influences these parts. Information flows bidirectionally between smaller and larger systems as well as rhizomatic contagion. When this bidirectionality of information flow and understanding of role is compromised, for whatever reason, the system begins to break down: wholes no longer recognize their dependence on their subsidiary parts, and parts no longer recognize the organizing authority of the wholes. Cancer may be understood as such a breakdown in the biological realm.
Ken Wilber comments that the test of holon hierarchy (e.g. holarchy) is that if all instances of a given type of holon were removed from existence, then all those holons of which they were a part must necessarily cease to exist too. Thus an atom is of a lower standing in the hierarchy than a molecule, because if you removed all molecules, atoms could still exist, whereas if you removed all atoms, molecules, in a strict sense would cease to exist. Wilber's concept is known as the doctrine of the fundamental and the significant. A hydrogen atom is more fundamental than an ant, but an ant is more significant.
A significant feature of Koestler's concept of holarchy is that it is open ended both in the macrocosmic as well as in the microcosmic dimensions. This aspect of his theory has several important implications. The holarchic system does not begin with strings or end with the multiverse. Those are just the existing limits of the reach of the human mind in the two dimensions at the present time. Those limits will be crossed later on because they do not encompass the whole of reality. Popper (Objective Knowledge) teaches that what the human mind knows and will ever know of truth at a given point of time and space is verisimilitude - something like truth, and that the human mind will continue to get closer to reality but never reach it. In other words, the human quest for knowledge is an unending journey with innumerable grand sights ahead but with no possibility of reaching the journey's end. The work of modern physicists designed to discover the theory of everything (TOE) is reaching deep into the microcosm under the assumption that the macrocosm is eventually made of the microcosm. This approach falls short on two counts: the first is that the fundamental is not the same as significant and the second is that this approach does not take into account that the microcosmic dimension is open ended. It follows that the search for TOE will discover phenomena more microcosmic than strings or the more comprehensive M theory. It is also the case that many laws of nature that apply to systems relatively low in the hierarchy cease to apply at higher levels. M theory might have predictive power at the sub-atomic level but it will inform but little about reality at higher levels. The work of the particle physicists is indeed laudable but they should give the theory they are looking for another name. This is not to claim that the concept of holarchy is already the theory of every thing.
Types of holons 
Individual holon 
An individual holon possesses a dominant monad; that is, it possesses a definable "I-ness". An individual holon is discrete, self-contained, and also demonstrates the quality of agency, or self-directed behavior. The individual holon, although a discrete and self-contained whole, is made up of parts; in the case of a human, examples of these parts would include the heart, lungs, liver, brain, spleen, etc. When a human exercises agency, taking a step to the left, for example, the entire holon, including the constituent parts, moves together as one unit.
Social holon 
A social holon does not possess a dominant monad; it possesses only a definable "we-ness", as it is a collective made up of individual holons. In addition, rather than possessing discrete agency, a social holon possesses what is defined as nexus agency. An illustration of nexus agency is best described by a flock of geese. Each goose is an individual holon, the flock makes up a social holon. Although the flock moves as one unit when flying, and it is "directed" by the choices of the lead goose, the flock itself is not mandated to follow that lead goose. Another way to consider this would be collective activity that has the potential for independent internal activity at any given moment.
American philosopher Ken Wilber includes Artifacts in his theory of holons. Artifacts are anything (e.g. a statue or a piece of music) that is created by either an individual holon or a social holon. While lacking any of the defining structural characteristics - agency; self-maintenance; I-ness; Self Transcendence - of the previous two holons, Artifacts are useful to include in a comprehensive scheme due to their potential to replicate aspects of and profoundly affect (via, say interpretation) the previously described holons. Artifacts are made up individual or social holons (e.g. a statue is made up atoms).
The development of Artificial Intelligence may force one to question where the line should be drawn between the individual holon and the artifact.
Heaps are defined as random collections of holons that lack any sort of organisational significance. A pile of leaves would be an example of a heap. Note, one could question whether a pile of leaves could be an "artifact" of an ecosystem "social holon". This raises a problem of intentionality: in short, if social holons create artifacts but lack intentionality (the domain of individual holons) how can we distinguish between heaps and artifacts? Further, if an artist (individual holon) paints a picture (artifact) in a deliberately chaotic and unstructured way does it become a heap?
Holon in Multiagent Systems 
Multiagent systems are systems composed of autonomous software entities. They are able to simulate a system or to solve problems. Holon may be viewed as a sort of recursive agent: an agent composed of agents which an agent at a given level has its own behavior as a partial consequence of these part's behaviors.
Janus Multiagent Platform is a software platform able to execute holons.
See also 
- Simon, Herbert A. (1969). The Sciences of the Artificial. Boston: MIT Press.
- Wilber, K. (2007). The integral vision. Shambhala Publications: Boston
- Luhmann, N. (1995). Social systems. Stanford University Press: California
- Calabrese, M. (2011). "Hierarchical-Granularity Holonic Modelling". Doctoral Thesis. University of Milan, Italy.
Further reading 
- Prigogine I and Stengers E (1984). Order out of Chaos. New York: Bantam Books.
- Koestler, Arthur (1967). The Ghost in the Machine (1990 reprint ed.). London: Hutchinson (Penguin Group). ISBN 0-14-019192-5.