Holism and emergent properties
One of about 300 papers at http://avancier.website. Last updated 16/03/2021 17:50 Copyright Graham Berrisford.
The terms “holism” and “emergent properties” have been used with a surprisingly large number of meanings. The discussion below offers definitions for “emergence”, and reviews the mess of other meanings.
"Were the engineer to treat bridge building by a consideration of every atom he would find the task impossible by its very size [therefore] studying very large systems by studying only carefully selected aspects of them is simply what is always done” Ashby 1956.
Part: a thing inside a system, be it an active structure (subsystem or actor) or a passive structure (material or information).
Wholeism (considering every conceivable aspect or element of a thing or situation) is impossible. For example, any model we make of a biological ecology excludes almost everything knowable about the physical reality it models.
Holism is sometimes interpreted to mean “zooming out”. The idea of zooming in and out is not specific to systems thinking. Basic thinking tools include a) testing a proposition or argument by taking it to an extreme b) making a case for the opposite of what you believe, and c) expanding and/or shrinking the boundary of what is being studied.
Zooming out. Sometimes, we look outside the boundary of a given system for the cause of an effect, and then rescope the system of interest to include that cause. Consider the dramatic flexing of the Tacoma Narrows bridge. At first, we might describe the flexing as an emergent property of the bridge. More accurately, it is an emergent property of a different and wider whole in which some parts of the bridge interact with a part of its environment - the wind.
Zooming in. Designers are taught to look first at a required system from the outside. Taking a "black box" view, they define the system’s inputs and outputs, and other effects or results of value to external actors. Then, they look inside the system and divide it into two or more parts (subsystems or actors) and design how those parts interact to produce the required state changes or outputs. If any part of the system could produce these “emergent properties” on its own, the rest of the design would be redundant.
Whether a system designer approaches the task by top-down system decomposition, or by identifying atomic parts to be assembled (bottom up) into a system, there is always an atomic level of system definition. The atomic parts are readily obtainable, or another designer's problem, or indivisible agents such as human actors.
Some systems thinkers deprecate reductionistic thinking, but this seems to be a mantra with no clear meaning, and misleading.
Reductionism 1: studying one or more parts of a thing on its own. You can't take a holistic view of a thing until you have identified two or more parts of it. And clearly, people do zoom in and out; they decompose and compose systems as they see fit. So, one person’s part is another person’s whole. A general practitioner may see a patient’s heart as an atomic part of a whole body. A heart surgeon sees the regular beating of the heart as an “emergent property” of particular muscles and valves interacting inside a heart.
Reductionism 2: looking for the part responsible for some behavior of a whole. This is perfectly normal. The interface to a whole word processor includes the interface to one part, a spell checker. The part fulfils one function of the whole. It is not responsible for all the functions of the whole, else the rest of the whole would be redundant. Moreover, it can be removed with little or no impact on the whole. (Consider also the spleen, gall bladder and appendix of a human.)
Clearly, there are times when looking inside a subsystem, studying a part on its own and fixing or removing it, is necessary and beneficial. You might even study the psychology of problematic human actors.
“We presently "see" the universe as a tremendous hierarchy, from elementary particles to atomic nuclei… to cells, organisms and beyond to supra-individual organizations… We cannot reduce the biological, behavioral, and social levels to the lowest level, that of the constructs and laws of physics.” Bertalanffy 1968
This table (bottom to top) presents a history of the universe from the big bang to human civilisation.
Elements or actors
Information encoded in writings
Science and enterprise
Humans in groups
Information encoded in speech
Teaching and logic
Animals in groups
Information encoded in signals
Parenting and copying
Animals with memories
Sense, thought, response
Sense, response. Reproduction
Matter and energy
Bertalanffy’s writings were much influenced by thinking at the biological level, thinking of a biological organism as a system. Another biologist and systems thinker Maturana, observed that “knowledge is a biological phenomenon”. In other words, before life, there was no knowledge, description or model of reality.
People use hierarchies to make sense of the messy network of things that exist in reality. At each level of thinking, hierarchies may be defined. Consider, in biology, the hierarchy of body, organ, cell, organelle, organic chemical. In physics, the hierarchy that descends from galaxies through solar systems down to atomic particles.
Emergence: the appearance of properties in a higher or wider thing that emerge from coupling lower or smaller things.
The term has several more specific meanings. With reference to the levels in the table above, we can identify three kinds of emergence.
The emergence by evolution of higher levels over time. Some see the table above as a hierarchy in which complexity increases from bottom to top. However, we usually compare the complexity of things at one level, ignoring lower levels. After all, every social group (simple or complex) has the same complexity at the level of molecules.
The everyday emergence of higher-level phenomena from lower-level phenomena. Somehow, conscious thought emerges from electrical activity in the brain. However, we don’t attempt to explain thought in terms of electrons, or explain a baseball game in terms of molecules; or explain Facebook in terms of the hardware components and radio waves it depends on. We usually model a system at one level of thinking, or adjacent levels.
The everyday emergence of effects from interactions between things at one level.
“that a whole machine should be built of parts of given behavior is not sufficient to determine its behavior as a whole: only when the details of coupling are added does the whole's behavior become determinate. ” Ashby 1956
Consider the effects or results that emerge from interacting things in the examples below. In each case, the interaction between things produces an outcome the things cannot produce on their own.
· The force produced by a wind passing over a sail.
· The progress of a rider on a bicycle.
· The V shape of three geese in flight.
· The shimmering of a school of fish.
· The price of fish that emerges when customers and suppliers strike a deal.
System design is holistic by definition, since the requirement for a to-be designed system is to produce emergent properties or effects. Given a system to design, the designer must start with those. And when a designed system produces unexpected effects, we call them "unintended consequences".
Not only is emergence is used with various meanings above, it is also wrongly presumed to imply a multitude of actors or agents, or unpredictable or complex behavior.
Emergence does not mean a system behaves in a surprising or unpredictable way. Natural systems often produce results or effects that appear surprising or mysterious. At least, they appear so, until you know how they work. By contrast, designed systems are intentionally designed to produce specified results or effects.
Emergence does not mean a system is complex in any normal sense of the term. Because in systems thinking, we necessarily ignore the internal complexity of the “atomic parts”. In studying a rider riding a bicycle, we ignore the complexity in the biology of the rider, and the motions of the ball bearings in the wheel hubs. In the discussing the orbits of planets in the solar system, we ignore the complexity in the composition and atmosphere of each planet.
You might say that when a system changes from one state to the next, the new state emerges from the performance of its regular behaviors. You might say that when a system mutates, the new generation of the system emerges from a change made to the variables, roles of the system. Arguably, these are abuses of the term
Emergence of observable mutations (novel variables, roles or rules)
This is a very different kind of emergence. If it happens continually in an ad hoc, uncontrolled or unpredictable way then there is no system - only an ever-unfolding process.
Suppose the actors in a social network continually invent and copy new activities in an uncontrolled or unpredictable way. Then there is no recognisable, describable or testable system. It is impossible what possible actions an actor might perform, or even which actors are playing roles in which systems. We can observe people communicating and doing things, in a social network. but to call that a “system” is useless if not meaningless.
Emergent does not mean surprising.
A bicycle and its rider interact in the emergent behaviour of smooth forward motion. But the emergent behaviour is not at all surprising. It is the very requirement for which a bicycle is designed.
Emergent does not mean unwanted or unexpected.
If you had never seen a bicycle ridden before, the forward motion of bicycle and rider would be a surprise. However, that motion was wanted and expected by the bicycle designer. The requirements for any designed system must include emergent properties.
Emergence does not necessarily mean the interacting components remain decoupled.
Two hydrogen atoms and one oxygen atom may interact to form a molecule of water. The three atoms are well-nigh inextricably bound in one emergent structure.
Emergence does not necessarily mean a whole is more complex that its parts.
It can instead mean the emergence of a simple abstraction from complexity. In physics, emergence describes the abstraction of simple macroscopic laws and concepts from complex microscopic laws and concepts. E.g. the simple laws of thermodynamics emerge from the complex laws that govern the interactions between component particles.
People speak of complexity (and emerging properties) appearing when you consider how parts interact in a whole. Yet the complexity of a system depends on the level of abstraction in its description. The coarser-grained the atomic actors and activities, the simpler the system.
Complexity can also appear when you look inside the atomic elements of a system. The deeper you immerse yourself in how something works, the more complex it gets. E.g. a game of noughts and crosses (tic tac toe) is a social system with very simple rules; a game of chess has somewhat more complex rules. Far more complex than either is the biology of a human actor, or the electronics of a computer actor, playing a role in the game.
The most basic kind of emergence may be defined thus.
Emergent property: a behaviour or structure of a whole that depends on interactions between its parts.
E.g. the forward motion of a cyclist on a bicycle, or the V shape of a flight of geese.
Typically, an emergent property is not found in any one part of a system, or predictable by studying a part in isolation from others.
(The exception is systems with homogenous populations, like a school of fish, where a subset of the whole may have the same properties as the whole.)
Properties of a whole are said to “emerge” when two or more parts, in cooperation, do something each part cannot do on its own.
If you expand the system boundary, include
more parts and interactions, then new system properties will emerge.
If you shrink the system boundary, exclude some parts and interactions, then some system properties will be lost.
That is such a simple and obvious idea it can be taken as axiom of system design.
It is well-nigh axiomatic that:
· Systems can interact: output from one can be input to another.
· Systems can be nested: one can be a part or subsystem of another.
· Systems can overlap: they can share the same part or subsystem.
· The smallest possible system (or subsystem) encapsulates one primitive or atomic action.
· An event that is external to smaller system is internal to a wider system (and vice-versa).
· The emergent properties of a small system are ordinary properties of any wider system it is a part of.
It may seem obvious to you that parts in connection can do things a part cannot do on its own.
It is no surprise to system designers and engineers, because emergent properties are very purpose of system design.
Designers start with a vision that encapsulates the system of interest; they define properties required of whole,
The purpose of system design is to obtain properties from the whole that cannot be obtained from isolated parts.
Suppose design proceeds by dividing the system into parts A and B.
If part A could do what the whole system can do, you wouldn't bother to design part B or include it in the system.
A second kind of emergence is discussed below.
Emergence of “higher-level” phenomena: a logical behaviour or structure is derived from more physical or concrete behaviour or structure.
E.g. the emergence of conscious thought from electrical activity in the brain.
The terms "higher" and "lower" usually refer to levels of abstraction.
But there are several kinds of abstraction, including:
1. Abstraction of a “higher” composite from one or more levels of decomposed parts
2. Abstraction of a “higher” client from one or more levels of servers
3. Abstraction of a “higher” generalisation from one or more levels of specialisations
4. Abstraction of a “higher” idealisation from one or more levels of more physical concrete forms.
The basic definition of emergence above relates to the first: to abstraction of a composite from decomposed parts.
Parts do something when interacting in composite that they cannot do on their own.
Abstraction of higher phenomena is related to the second kind of abstraction, of a client from servers.
Consider for example, the step by step process by which the text you are reading arrived at you - via the internet.
1. I intend to convey a meaning.
7. You decode and extract a meaning
2. I encode my meaning in text
6. You read the text I wrote
3. Several levels of translation occur
5. Several levels of translation occur
4. Matter/energy variations are transmitted in wires and radio waves.
You might well say the meaning of the text you are reading (7) emerged from lower level matter/energy phenomena (4).
You might also say the lower level phenomena (4) emerged from higher level phenomena (1).
Indeed, the terms "higher" and "lower" feel somewhat subjective.
There is a series of translation steps, but one could as well draw the same communication stack diagram upside down.
I suspect that is all you need to know about emergence, but read on for discussion of other views and concepts.
“In one interpretation, holism is a methodological thesis, to the effect that the best way to study the behavior of a complex system is to treat it as a whole, and not merely to analyze the structure and behavior of its component parts.” https://plato.stanford.edu/entries/physics-holism
This definition might be interpreted as encapsulation rather than holism
You can draw a diagram to show flows into and out of a system, showing nothing of what is inside the system.
This “black box” diagram is not a holistic view, because it does not show how parts cooperate to the benefit of the whole.
Also, you cannot see a property of a whole as being emergent until you see its parts (in isolation) do not have the same property.
If you can see any parts, then you cannot see whether a property of the whole requires more than one part.
So you cannot describe any property of the whole as “emergent”.
“Methodological holism stands opposed to methodological reductionism, in physics as well as in other sciences.” https://plato.stanford.edu/entries/physics-holism
Hmm… you cannot study how parts cooperate in a whole with first identifying the parts - which implies analysing that whole in a reductionist view of the world.
A metaphysical thesis?
“Alternatively, holism may be taken as a metaphysical thesis.
There are some wholes whose natures are simply not determined by the nature of their parts.
But it is a certain variety of metaphysical holism that is more closely related to nonseparability.” https://plato.stanford.edu/entries/physics-holism
Here, holism is not a metaphysical thesis.
It means only that a whole is more than the sum of its parts, because the whole depends on cooperations between parts.
Holistic: considering how the parts of a whole are related; taking a view that addresses how parts cooperate to the benefit of the whole.
Systemic: relating to the whole rather than a part; reaching throughout the whole. (E.g. A systemic drug, disease, or poison reaches and has an effect on the whole of a body.)
Parts are interrelated so strongly they cannot exist independently?
The peripheral parts of a biological entity cannot exist independently (your arm dies if it is cut off).
But the core parts can live without periperhals (you can live without your limbs).
A bicycle and its rider exist independently of teach.
IBM could be subdivided into smaller independent businesses.
A part cannot be understood without reference to the whole?
Systems are recursively composable and decomposable.
You can understand a system or subsystem at any level of decomposition you choose to.
And note that a part can be greater than its role in a system.
E.g. a person is greater than his/her role in a bicycle-rider system.
Emergent properties emerge from the interaction of actors or components in a wider or higher process.
These discrete things
When coupled in a process can
Person and bicycle
Bridge and wind
Fly in a V shape
Person, screwdriver, screw and wood
Drive the screw into wood
Board, CIO and EA team
Standardise and integrate business systems
A person and a bicycle are discrete entities.
The process of pedalling the bicycle produces the emergent properties of balance and motion.
In this case, the emergent property was deliberate and wanted.
The famous wobble of the Tacoma Narrows Bridge (which led to its collapse) has been called an emergent property of the bridge.
Wrong! It was an emergent property of the wider process in which the bridge and the wind were interacting components.
In this case, the emergent property was accidental and unwanted.
Bertalanffy said the whole is more than the sum of its parts.
The whole system has properties that “appear as new or emergent” because the parts do not have those properties.
Thus, he implied that properties emerge from the interaction of actors/components previously considered separately.
Laszlo and Krippner (1998) distilled what many systems thinkers agree about the concept.
“An emergent property is marked by the appearance of novel characteristics exhibited on the level of the whole ensemble, but not by the actors/components in isolation.” Laszlo and Krippner
The also defined the so-called “removal of parts” test.
“Emergent properties are lost when the system breaks down to its components.
The property of life, for example, does not inhere in organs once they are removed from the body.
“When a component is removed from the whole, that component itself will lose its emergent properties.
A hand, severed from the body, cannot write, nor can a severed eye see.” Laszlo and Krippner
The test is misleading because:
· you can indeed remove parts from a homogenous system (remove a fish from a school of fish) without changing the qualitative properties of the system.
· you can remove some parts from a multi-purpose system (remove a font from a word processor) without most observers noticing.
· writing is not in the hand, seeing is not in the eye, these emergent properties were never in the parts; they required processes that connect those parts to the brain.
Ashby (1956) was clearest
“Parts can be coupled in different ways to form a whole.
The defining of the component parts does not determine the way of coupling.
From this follows an important corollary: that a whole machine should be built of parts of given behaviour is not sufficient to determine its behaviour as a whole.
Only when the details of coupling are added does the whole's behaviour become determinate.”
In short, properties emerge not from merely aggregating parts, but from processes (not found in the parts) that couple the parts.
Usually, an emergent property is a property of a whole that depends on and is derived from interactions between its parts
Emergent properties are the very purpose of engineering and system design.
Of course, a subsystem cannot do everything that whole system can do – else the rest of the system would be redundant.
However people use the term “emergent property” with many other meanings, such as:
· a surprising or mysterious property
· an essential-to-purpose property
· an accidental property.
· a run-time-only property.
Perhaps the most interesting is a property that is mysterious, it is puzzling to an observer who knows the system’s parts
Especially when simple rules followed by individual actors/components create what appear to be complex behaviors.
Read the analysis below for a deeper exploration of such alternative meanings.
Surely it is obvious that two subsystems cannot do everything (separately) that they can do when coupled in a wider system? Barely worth saying?
Today, people use the term emergent property loosely, with various meanings, and sometimes to express a value judgement.
Questions to be explored below include:
· Does an emergent property differ from any other property?
· Does an emergent property require all of a system’s actors/components?
Before we explore these and other questions; a few notes on outputs and outcomes.
· Outputs are what a system produces; they are properties of the system.
· Outcomes are the result of what external entities do with outputs; they are properties of a wider system.
Possible outcomes include:
· Welcome outcomes.
· Unwelcome outcomes (cf. “the law of unintended consequences”)
· Unpredicted and welcome outcomes caused by external entities using system outputs in an unexpected way.
Think how you may use the term emergent property, then see if you can match your example(s) to the cases listed below.
Suppose you are asked to design a clock to help people tell the time.
Your clock must display an ever-changing number that stays in step with standard time.
You design and build the clock. What next?
You might bury your clock in the ground; it never works and is never used to tell the time.
Else, your clock could be used in various ways that illustrate a variety of behaviours some have called emergent properties.
A behaviour triggered by a condition or time passing
E.g. your clock’s alarm is set and goes off in response to the arrival of the set time.
That is a predictable, designed, deterministic behaviour that occurs when the systems advances to a new state.
A behaviour just now noticed by an observer
E.g. Long after buying the clock, the purchaser notices the clock has an alarm feature.
An unpredicted failure of the system
E.g. your clock’s parts do combine as they are designed to, but they work too fast, so display a number that almost never accurately represents the standard time.
An unwelcome outcome
E.g. your clock is used as the timer for a bomb.
That is an unpredicted and unwelcome outcome caused by an external entity using a system output in an unexpected way.
An unpredicted but welcome outcome
E.g. your clock is used to teach somebody how to tell the time.
A welcome output
E.g. In system testing, your clock’s parts cooperate to display a number that accurately represents the standard time.
A welcome outcome in operation
E.g. your clock is bought and used successfully to tell the time.
Note that time telling is a welcome outcome of the clock's output being used by a human being.
The time telling is a property of a clock-human system, not the clock alone.
Of course, different observers have different ideas about what the welcome or essential properties or purposes are. Take a car for example:
Wanting this outcome
Sees this as an essential property of a car
travel to destination
point to point transport
status in society
impress the neighbours
make a profit
avoid death on the road
wing mirrors, rear view
Below are more ways I have heard the term emergent property applied.
A run-time only property
A property observable in an operational system, but not found in its system description.
Perhaps a non-linear or “chaotic” outcome, like a hurricane, or a population crash in an ecology.
An essential-to-purpose property
A property necessary to a desired outcome.
This is subjective of course: What is essential in a daffodil, a word processor, a flight of geese, a bicycle, or IBM?
An accidental property
A side effect of what has been designed, not purposefully designed for, perhaps revealing of a design error.
A surprising property
One that (by comparison with reductionist inspection of a system’s parts) is unexpected or unpredictable.
A new-after-change quality
A qualitatively new property, an after-effect of an evolutionary step change, a new property of a new system generation not found in past system generations.
E.g. the first appearance of thorns on a plant.
A new-after-change quantity
A quantitative change in an old property, a higher or lower value of a variable.
E.g. Add gears to a bicycle, there is a reduction in pedalling effort.
Merge two companies, their turnover (separately or combined) goes up or down.
Naturally, when two or more systems cooperate, the wider system may exhibit properties the subsystems cannot exhibit on their own.
And system designers start by defining the properties that will emerge when subsystems are assembled and deployed.
The reason for designing any system is to obtain properties that cannot be obtained from its isolated parts.
All said above seem to imply we know what system we are talking about.
And the term “emergent property” implies there is a system to which the property is attached.
Yet, when the term is used, it is often not clear what "system" is being discussed.
So which emerges first? The property or the system?
Sometimes the logic goes as follows.
Something happened we didn’t expect; we’ll call that an "emergent property".
Then we’ll define whatever chain of events produced the property to be a "system".
Are you familiar with "root cause analysis diagrams" or “cause and effect diagrams”? You can find some on the web.
People draw such a diagram to uncover and show the curious collection of entities and events that have accidentally conspired to cause an unexpected thing to happen.
They might look for root causes under pre-defined headings such a people, management, materials, equipment, measurement and environment.
You can of course characterise an event (a problem, an incident, an accident) as “emergent” and then analyse to find some actors/components that interacted in some processes to cause that event.
This root cause analysis can reveal a “system” that nobody was aware of before; it was never designed; its scope was defined only in retrospect.
Lo, a system emerges from analysis of a property! However, this system was not understood to be a system in advance of the event happening.
And using this logic, everything that ever happens can be subjected to root cause analysis and then described as the emergent property of its own unique system.
Would von Bertalanffy or Ashby have called this accidental conspiracy a "system"? Wouldn’t they rather call it a disorderly and unsystematic process?
Suppose we return to the simple definition that an "emergent property" is a feature of a whole system but not a part of it.
The following two test questions have been suggested to distinguish an emergent property from any other property.
The idea is that if the answers are “no”, then the property can be called emergent.
Q) Can you observe the property if less than the whole system is present? Can you remove parts and still observe the property?
Below are four test cases.
The first two pass the test; the last two fail the test; but it is possible to manipulate the analysis so they pass also.
Example 1: The Tacoma narrows bridge, which wobbled so much in a strong wind it collapsed.
Q) Can you observe the property if less than the whole system is present? Can you remove parts and still observe the property?
No. You need both the bridge and the wind.
The emergent property was not the resonant frequency of the bridge, or the collapse.
It was the wobble caused by a process involving the bridge and wind as interacting actors/components in a wider system.
Example 2: a rider’s balance on a bicycle.
Q) Can you observe the property if less than the whole system is present? Can you remove parts and still observe the property?
No. You need to both the bicycle and the rider.
Balance is not a property of the bicycle alone.
For the property to exist and be observed, you have to widen the system boundary to embrace bicycle and rider.
Example 3: using groupware to send an email
Yes, if you can remove parts of your email system that do not contribute to sending an email.
Example 4: the speed of a car
Yes if you remove parts of the car that do not contribute to its motion.
Remove one wheel from my wagon, and I still keep rolling along.
In the last two examples, you could define the property as an emergent property by logically-bounding a system that contains only those parts necessary to enable the property.
But that is a self-referential definition, which you could apply to any property.
Again, using this logic, everything that ever happens can be analysed by root cause analysis and then described as the emergent property of its own unique system.
A heterogenous system comprises many kinds of part.
A homogenous system comprises many parts of the same kind, in which each subsystem instance follows the same rules.
In some homogenous systems, a subsystem instance has very simple rules to follow.
But when subsystems combine to act in a group, the group may exhibit a behaviour that looks complex – deceptively so – since the part’s rules are so simple.
Such mysterious emergence seems relatively normal in biological, evolved, systems.
Consider the V shape of a flight of geese.
Apply the emergent property test questions.
Q) Can you observe the property if less than the whole system is present? Can you remove parts and still observe the property? Yes, provided at least three geese remain in the flight.
You can remove some geese and the flight looks much the same.
The example fails the test for an emergent property.
Yet it is often used as an example, surely because it has an element of mystery about it.
Here are some quotes from “The demystification of emergent behavior” Gerald E. Marsh, Argonne National Laboratory (Ret) 5433 East View Park, Chicago, IL 60615
· “The concept has been used to justify all sorts of nonsense.
· It is really more open than simply the combination of two "subsystems" whose design and interfaces are specified before they come together.”
· “there is [no] universally acknowledged definition of emergence.” [However it usually means] "Emergent behavior transcends a mere increase in the behavioral degree of complexity."
· “Complex behavior can arise from the underlying simple rules”
· "The emergent properties do not in any transparent way derive from the underlying rules governing the interaction of the system’s components.”
· "Associated with this phenomenon is a sense of the mysterious.”
1 Alex Ryan, “Emergence is coupled to scope, not level”, arXiv:nlin/0609011 v1 (2006).
2 Alexander K. Dewdney, “Insectoids Invade a Field of Robots” Scientific American (July 1991).
3 M. Mitchell Waldrop, “Fast, Cheap, and Out of Control”, Science 248 (1990): 959-961.
4 Randall D. Beer, Hillel J. Chiel and Leon S. Sterling, “An Artificial Insect”, American Scientist 79 (1991): 444-452.
5 Deborah M. Gordon, “The Development of Organization in an Ant Colony”, American Scientist 83 (1995): 50-57.
6 Arun V. Holden, Ed., “Chaos” (Princeton University Press, Princeton 1986).
8 Epigenetics, Nature Insight: Nature 447 (2007): 395-440.
9 Arthur Köestler, “The Ghost in the Machine” (The Macmillan Co., New York 1968).
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