We began this series with a modest linguistic decision.
When we asked what a particle does, we said that by "function" we meant nothing mysterious: simply what it does.
No intention.
No purpose.
No cosmic plan.
No little electron sitting in the atom wondering what career it ought to pursue.
And yet, as the series has unfolded, the word has become increasingly difficult to keep quite so modest.
An electron can participate in chemical bonding.
A photon can participate in the transmission of differences.
A receptor can detect those differences.
A molecular network can transform them.
An organism can use them to guide behaviour.
At each stage, something is doing something.
But the relationship between what a component can do and what a larger system depends upon it to do seems to become increasingly interesting.
We have therefore arrived at the question that has been waiting for us:
When does a capacity become a function?
The answer, I think, is not that function requires purpose.
It is that function emerges when a capacity becomes consequential within an organised system.
And that gives us a surprisingly natural bridge from physics to biology.
Capacity comes first
Let us begin with the simpler notion.
A capacity is something a physical system can do under appropriate conditions.
An electron has the capacity to interact electromagnetically.
A molecule has the capacity to react with another molecule.
A crystal has the capacity to conduct or resist electrical current depending upon its structure.
A photon has the capacity to interact with matter in particular ways.
None of these capacities needs a purpose.
They are simply consequences of the physical organisation of the system.
This is important because it gives us a natural starting point.
The universe does not need to contain functions in advance.
It needs to contain capacities.
Once capacities exist, interactions can occur.
Once interactions occur, organisations can form.
And once organisations form, some capacities can become especially important to what those organisations do.
That is where function enters.
A function is a capacity with a job
The word "job" is dangerous here because it sounds intentional.
But it provides a useful intuition if we immediately remove the intention.
Suppose a membrane allows certain molecules to pass while restricting others.
The membrane therefore has a capacity for selective permeability.
If that selectivity is essential to maintaining the cell's internal conditions, we can say that selective permeability is one of the membrane's functions.
The membrane does not intend to regulate the cell.
Nothing needs to have assigned it the job.
Its physical structure simply contributes to the organisation's ability to persist.
Function, in this sense, is a role within an organisation.
We could formulate it as:
A capacity becomes a function when an organised system depends upon that capacity to maintain or realise some of its characteristic activities.
This definition is deliberately relational.
The function is not merely in the component.
Nor is it merely in the system.
It lies in the relation between them.
The heart does not have a purpose because we say it does
Consider the heart.
We say that its function is to pump blood.
That sounds teleological.
But the claim need not mean that the heart was created for pumping blood.
It means that the heart's activity contributes to maintaining the circulation on which the organism depends.
If the heart stops pumping, the organism rapidly loses many of its capacities.
The heart's pumping is therefore not merely something it happens to do.
It is something upon which the organisation of the organism depends.
This gives us a useful distinction:
A function is not necessarily what something was made to do. It can be what an organisation has come to depend upon it doing.
This distinction will become increasingly important.
It allows us to speak naturally about functions in nature without smuggling intention into the explanation.
Dependence changes everything
Imagine two components in a physical system.
Component A produces an effect on component B.
That is ordinary causation.
Now imagine that the system's continued organisation depends upon A's effect on B.
We have something more.
The relation has become structurally significant.
If A's behaviour changes, the system's characteristic activity changes.
If A disappears, the system may cease to function.
The causal contribution has become part of the organisation's architecture.
This suggests another formulation:
Function is causation organised into dependence.
That may be too compressed to serve as a complete definition, but it captures something important.
The difference between "A causes B" and "A functions to produce B" is not necessarily a difference in physical mechanism.
It is a difference in organisational context.
The electron again
Now consider our original electron.
Does an electron have a function?
We need to be careful.
The electron does not have a function in the biological sense.
But electrons certainly have capacities whose participation is indispensable to higher-level organisations.
The electronic structure of atoms enables chemical bonding.
Chemical bonding enables molecules.
Molecules enable chemistry.
Chemistry enables life.
So can we say that the electron's function is to form molecules?
Not quite.
That would confuse a physical capacity with a biological role.
The electron does not exist in order to make molecules.
But its physical capacities participate in the organisation from which molecular functions emerge.
This distinction is valuable.
At the fundamental level, we can speak primarily of capacities.
At higher levels, those capacities become incorporated into organised roles.
Function is therefore not something we need to attribute uniformly at every level.
It is something that can emerge through organisation.
The danger of looking backwards
There is a seductive mistake here.
We know that electrons participate in chemistry.
We know that chemistry makes life possible.
We know that life contains organisms with functions.
It is therefore tempting to look backwards and say:
The electron's function is to make chemistry possible.
But that is history written backwards.
The electron did not have a future appointment as the foundation of biology.
Its capacities existed long before organisms.
What happened was that later organisations discovered ways of exploiting those capacities.
This is a recurring pattern in nature.
A capacity can exist before anyone—or anything—has a use for it.
Later, a new organisation can arise in which that capacity becomes consequential.
Evolution is extraordinarily good at this.
It takes whatever the world happens to provide and explores what can be built from it.
Evolution turns possibilities into functions
This is perhaps where biological function becomes distinctive.
Natural selection does not merely preserve structures.
It preserves consequences.
Suppose a variation gives an organism a structure that happens to improve its ability to detect food.
If that improvement affects survival or reproduction, organisms possessing the variation may become more common.
Over generations, the structure can become elaborated.
Its contribution to food detection becomes increasingly integrated into the organism's organisation.
At some point, it becomes entirely natural to say:
This structure functions in detecting food.
Nothing intentional has happened.
But something historical has.
A capacity has been stabilised by selection because of the difference it makes to the organism.
Function has acquired historical depth.
Function has a history
This is why biological functions are not quite like ordinary physical capacities.
A rock can conduct heat.
A piece of metal can conduct electricity.
But we would not normally say that conducting electricity is the function of an arbitrary piece of metal merely because it happens to do so.
Why not?
Because the metal's organisation has not necessarily been shaped around that contribution.
An engineered wire is different.
Its electrical conductivity has been selected and maintained because of the role it plays in a larger system.
The wire has a function.
The raw metal has a capacity.
This distinction becomes even clearer in biology.
The heart's pumping is not simply an effect.
It is a contribution embedded in an organisation that has been historically shaped around maintaining circulation.
The difference between capacity and function is therefore partly a difference between:
what can happen
and
what the organisation has come to depend upon happening.
Exaptation: nature reuses capacities
But evolution immediately complicates this picture.
A structure can acquire a new function that was not responsible for its original evolution.
Feathers provide a famous example.
They are deeply associated with flight, but their evolutionary history appears to have involved earlier functions such as insulation and display.
Flight could therefore emerge from a structure whose earlier organisation served other roles.
This is a wonderful example of our central theme.
A capacity can exist before its eventual function.
A structure can participate in one ecology of possibilities and later become incorporated into another.
Evolution does not have to invent every capacity from scratch.
It can recruit existing capacities into new organisations.
This is one reason biological evolution can be so creative.
It is continually reorganising what already exists.
The world is full of latent possibilities
The distinction between capacity and function therefore gives us a new way of understanding potential.
A capacity is a kind of latent possibility.
It says:
under suitable conditions, this system can participate in something.
A function says something stronger:
within this organisation, that capacity has become systematically consequential.
This means that the world may contain enormous numbers of capacities that are never incorporated into functions.
There may be physical possibilities that no organism ever exploits.
There may be chemical reactions that no living system ever discovers.
There may be mathematical structures in physical systems that no intelligence ever notices.
Possibility is therefore much larger than function.
Function is possibility recruited by organisation.
That seems to me a particularly fruitful way of putting the relationship.
Recruitment is the key
The word recruitment is useful because it avoids intention.
An organisation can recruit a capacity without anyone deciding to recruit it.
Evolution can recruit a physical property.
A cell can recruit a chemical gradient.
A nervous system can recruit a sensory difference.
A social system can recruit a signal.
A language can recruit a sound pattern.
In each case, something that was already physically possible becomes incorporated into a new pattern of organisation.
And once incorporated, its consequences change.
The same capacity can therefore have different roles in different systems.
A molecule can be a nutrient in one context and a signal in another.
A movement can be locomotion in one context and communication in another.
A sound can be noise in one context and a word in another.
A photon can be heat-producing radiation in one context and visual information in another.
Function is therefore contextual without being arbitrary.
The surrounding organisation determines what role a capacity can play.
Function is relational
We can now state this more strongly.
A function is not a hidden property waiting to be discovered.
It is a relation between:
- a capacity,
- an organisation,
- and a set of consequences that matter to that organisation.
The same physical capacity can therefore support different functions in different contexts.
Consider an electrical current.
In one system it may heat a wire.
In another it may drive a motor.
In another it may carry a communication signal.
In a neuron it participates in information processing.
The underlying physics is continuous.
The functions are not.
They emerge from the organisation in which the physical process is embedded.
This is exactly what we should expect if function is a higher-level relational property.
The function of the photon
We can now return to our photon with greater precision.
Does a photon have the function of carrying information?
Not inherently.
It has physical capacities that make information transmission possible.
Within an optical communication system, particular patterns of electromagnetic radiation can function as signals.
Within an eye, particular interactions with photoreceptor molecules contribute to visual detection.
Within photosynthesis, photons provide energy that drives chemical processes.
Within a solar cell, they contribute to electrical power generation.
The same basic physical entity participates in different organisations and thereby acquires different functional roles.
The photon has not changed its fundamental nature.
The organisation around it has changed.
This is perhaps the clearest illustration yet of the principle we have been developing:
Function belongs to an organised relation, not to an isolated object.
Function can migrate
Once we see function this way, something else becomes visible.
Functions can migrate across physical substrates.
A biological system may detect light using one molecular mechanism.
Another organism may detect it using another.
A camera can perform a roughly analogous detection using semiconductor components.
A telescope can detect electromagnetic radiation using still other mechanisms.
The physical implementation changes.
The functional role remains recognisable.
This tells us something profound.
Function is often more abstract than its physical implementation.
There can be many ways of realising the same functional relation.
And conversely, one physical structure can participate in several functions.
This is one reason why higher-level sciences can identify patterns that remain invisible at the level of elementary particles.
They are tracking organisational roles, not merely constituents.
A function can become a constraint
There is another important consequence.
Once a system depends upon a function, that dependence constrains what can happen.
A heart must maintain a certain range of pumping behaviour.
A membrane must regulate exchange.
A metabolic pathway must preserve certain relationships.
A nervous system must maintain enough integrity for signals to propagate.
The function therefore creates a kind of internal norm.
Some states are compatible with the system's organisation.
Others threaten it.
This gives us something approaching biological normativity without invoking anything supernatural.
A heart that stops pumping is not merely "different."
It is failing relative to the organisation of the organism.
A membrane that becomes freely permeable is not merely exhibiting another possible state.
It has lost a capacity on which the cell depends.
Function therefore creates the possibility of failure.
And failure is a surprisingly important concept.
Failure reveals function
One of the best ways to discover a function is to ask what happens when it is absent.
If removing a component has no effect, perhaps its contribution was not functionally significant.
If removing it destroys the organisation's characteristic activity, we have evidence that its capacity was functionally important.
The test is not infallible, because biological systems are often redundant.
But conceptually it is powerful.
A function becomes visible through dependence.
We often fail to notice functions precisely because they work so reliably.
We notice the heart when it fails.
We notice vision when it is lost.
We notice infrastructure when it breaks.
Normal functioning hides the organisational dependencies that sustain it.
Failure makes them visible.
Function creates norms without intentions
This allows us to solve an old philosophical problem.
How can nature contain things that are "supposed" to happen without anyone intending them?
The answer may be:
organisation can generate norms.
If an organism depends upon a particular process, then some states of that process are compatible with the organism's continued organisation and others are not.
The norm is therefore generated internally by the system's mode of existence.
No external purpose needs to be imposed.
The heart is not "supposed" to pump because the universe decreed it.
It is supposed to pump, in the functional sense, because an organism organised around circulation cannot persist without circulation.
Function is therefore not necessarily a mysterious property of matter.
It can be a consequence of organised dependence.
But not every useful effect is a function
We should nevertheless be cautious.
Suppose a bird happens to provide shelter for an insect.
The insect benefits.
Does shelter provision therefore constitute a function of the bird?
Probably not.
The bird's body may have effects that other organisms exploit without those effects being integrated into the bird's own organisation.
This distinction is important.
A function is not simply any consequence that happens to be useful to someone.
It is a contribution that belongs to the organisation in question.
Likewise, if a person uses a stone as a paperweight, the stone has not thereby acquired the biological function of holding down paper.
The distinction between effect and function therefore remains important.
Function requires organisational relevance.
And yet functions can be reassigned
But the story does not end there.
Once an organism discovers a new use for something, a new functional role can emerge.
A stick can become a tool.
A sound can become a word.
A gesture can become a signal.
A naturally occurring object can become part of a technological system.
Now we have a new source of function.
Not natural selection alone, but intentional and cultural organisation.
Human beings can deliberately recruit capacities into new systems.
We can make a stone into a hammer.
We can make electromagnetic radiation into a communication channel.
We can make silicon into a computational substrate.
We can make language into an instrument of science.
Culture therefore becomes an extraordinarily powerful mechanism for converting capacities into functions.
And once again, the general pattern survives:
existing capacity → new organisation → new functional role → new possibility.
Technology accelerates the process
Technology makes the distinction especially visible because we can watch it happen.
Silicon has certain electrical properties.
Those properties make transistors possible.
Transistors make logic circuits possible.
Logic circuits make computers possible.
Computers make new forms of information processing possible.
Networks make distributed computation possible.
Artificial intelligence systems make new forms of interaction between humans and machines possible.
At every stage, previously existing capacities are recruited into new organisations.
The resulting system acquires functions that could not have been attributed to its components in isolation.
The transistor does not "compute."
A sufficiently organised network of transistors can implement computation.
The distinction is crucial.
The function belongs to the organisation of capacities.
This is why reductionism keeps missing something
A complete physical description of every component of a computer would, in principle, describe everything physically happening inside it.
But it would not thereby make the concept of computation unnecessary.
We can ask:
What is the machine calculating?
What algorithm is it implementing?
What information is it representing?
What role does a particular component play?
These questions concern organisation.
They are not rival descriptions of the physical system.
They are descriptions of what the physical system is doing as an organised whole.
The same is true of biology.
A complete account of the molecules in a cell does not eliminate metabolism.
A complete account of neural activity does not eliminate perception.
A complete account of ink and paper does not eliminate language.
Higher-level descriptions remain legitimate because organisation creates new patterns of dependence.
Function is one of the names we give to those patterns.
Function as compressed history
There is perhaps another way to understand this.
When we say:
"The function of the kidney is to regulate the composition of the blood,"
we are compressing a vast amount of information.
We are saying that the kidney's structure, activity, developmental history and evolutionary history are organised around a particular contribution to the organism.
The word function is therefore a powerful explanatory shorthand.
It tells us which consequences are structurally significant.
It allows us to ignore countless irrelevant physical details while preserving the organisation that matters.
This is not an abandonment of physics.
It is an efficient way of talking about what physics has organised.
The ladder of functional emergence
We can now sketch a progression.
At the physical level:
capacities
At the structural level:
capacities become combined
At the ecological level:
capacities become mutually enabling and constraining
At the biological level:
some capacities become essential to self-maintaining organisation
At the evolutionary level:
those dependencies can become historically stabilised
At the semiotic level:
capacities can become involved in systems of significance
At the technological and cultural level:
capacities can be deliberately recruited into new organisations
The levels overlap.
They are not clean boxes.
But the progression reveals something important.
Function does not appear from nowhere.
It grows out of capacity organised into dependence.
Function and possibility
And now we can return to the central theme of the series.
A capacity says:
this can happen.
A function says:
this happening contributes to what this organisation can do.
That difference is subtle but fundamental.
The capacity concerns possibility.
The function concerns organised possibility.
The electron can participate in bonding.
In an atom, that participation contributes to molecular structure.
The molecular structure makes particular chemical reactions possible.
In a cell, some of those reactions become parts of metabolic functions.
The metabolism sustains the cell.
The cell's organisation makes sensory capacities possible.
Those sensory capacities make environmental information available.
The organism can use that information to act.
At each stage, possibility is being captured into organisation.
And organisation feeds possibility back into the world
There is one final step.
Once a system has a function, it can use that function to alter its environment.
An organism can move.
A plant can grow.
A beaver can build.
A human can construct.
A scientist can experiment.
A technological system can transform its surroundings.
The environment changes.
New capacities become available.
New functions can emerge.
So the process is not merely:
capacity → function.
It is:
capacity → organisation → function → environmental change → new possibilities → new organisation.
This is the dynamic we have been approaching throughout the series.
The world does not simply contain organisms.
Organisms become participants in changing the world of possibilities.
What, then, does a particle do?
We can finally return to our opening question.
What does an electron do?
It participates in physical interactions.
What does that make possible?
Atoms.
What do atoms make possible?
Molecules.
What do molecules make possible?
Chemical networks.
What do chemical networks make possible?
Living organisation.
What does living organisation make possible?
Detection, metabolism, reproduction, behaviour, communication.
What do these make possible?
Signs.
Symbols.
Culture.
Technology.
Science.
And perhaps new forms of organisation that we have not yet imagined.
At no point did the electron possess a purpose.
At no point did the photon know that one day it would help an organism see a tiger.
At no point did chemistry "aim" at life.
But the capacities were there.
And organisations discovered—or, better, came to embody—ways of exploiting them.
That may be enough.
The deepest lesson
Perhaps the most important distinction of the series so far is therefore this:
A capacity is what a system can do. A function is what that capacity becomes within an organisation that depends upon it.
The difference is not between physics and biology.
It is a difference that can occur within physics itself as organisation develops.
A physical capacity becomes part of a structure.
A structure becomes part of a network.
A network becomes part of a self-maintaining system.
A self-maintaining system becomes part of an ecology.
An ecological system becomes capable of detecting differences.
And eventually those differences can acquire significance.
Function is one of the bridges across these transformations.
It is the language we use when a mere possibility has become organisationally consequential.
The next question
But this leaves us with a fascinating puzzle.
If functions arise when capacities become embedded in organisations, then perhaps the most important thing about an organisation is not what it contains.
It is what it makes possible.
A cell is extraordinary not because it contains particular molecules, but because those molecules are arranged so that the cell can maintain itself.
An eye is extraordinary not because it contains photoreceptors, but because those components are organised so that the organism can detect and exploit differences in its environment.
A brain is extraordinary not merely because it contains neurons, but because their organisation permits forms of learning, prediction, action and representation.
And a culture is extraordinary because its organisation allows capacities to be transmitted, modified and recombined across generations.
We have therefore moved from asking:
What does a thing do?
to asking:
What does an organisation make possible?
That is a much larger question.
And perhaps it is the question that will carry us toward the heart of the series.
Because if physical reality is an ecology of capacities, and functions emerge from organised dependence, then perhaps the most remarkable fact about the universe is not that it contains things with properties.
It is that things can become organised in ways that make new kinds of things possible.
The physics of what becomes possible may therefore ultimately be a physics of organisation itself.
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