What Does It Mean to Exist?

To ask what exists seems simpler than asking what existence is.

A tree exists. A stone exists. A person exists. A galaxy exists.

At least these cases seem straightforward.

But the confidence begins to fade when the examples change.

Does a number exist?

Does a possibility exist?

Does a law of nature exist?

Does a fictional character exist?

Does a nation exist?

Does a memory exist?

Does a shadow exist?

Does a hole exist?

Does a thought exist?

Does the future exist?

And if these things exist, do they all exist in the same way?

The problem is not merely linguistic. The answer affects nearly every subject we will encounter in this series.

Physics asks what kinds of things the world contains.

Mathematics seems to speak about entities that are not located in space.

Computer science manipulates abstract structures that can be implemented in physical machines.

Biology depends on patterns such as species and genetic information.

Psychology studies mental states that are not visible in the same way as bones or cells.

Social life depends on entities such as money, borders, laws, universities, and governments whose existence depends partly on human agreement.

Before asking why anything exists, we should first ask what it means to say that something exists at all.

Existence Is Not a Property Like Color

Ordinary language encourages us to treat existence as if it were one more property.

A ball may be red, heavy, warm, and round.

We might then imagine adding “existing” to the list.

But something is strange about that move.

If we describe a creature as having wings, scales, intelligence, and immortality, adding the word exists does not suddenly place that creature in the world.

A description can be complete without having a real instance.

This insight became especially important in philosophy through debates about whether existence should be treated as a predicate—a property that objects possess.

Immanuel Kant famously argued that existence is not a real predicate in the ordinary sense. Saying that one hundred real coins exist does not add another descriptive feature to the concept of one hundred coins. It says that the concept is instantiated.

Modern logic handles this idea elegantly.

Instead of treating “exists” as an ordinary property, logic often expresses existence through quantification:

There is at least one thing for which this statement is true.

This seems technical, but it changes the question.

To say that unicorns exist would not mean that “existence” is a property attached to unicorns.

It would mean that reality contains at least one entity that satisfies the description unicorn.

So existence is deeply connected to what a theory allows us to quantify over.

What kinds of things are we willing to say there are?

That question is ontology.

Ontology: What Is There?

Ontology is the branch of philosophy concerned with being and the categories of things that exist.

A simple ontology might contain only physical objects.

A richer ontology might include:

• objects,
• properties,
• events,
• processes,
• relations,
• numbers,
• sets,
• propositions,
• mental states,
• possibilities,
• social entities.

The difficulty is that these categories do not behave alike.

A stone has a location.

A storm has a region and a duration, but its identity is more process-like than object-like.

A friendship exists between people but is not located inside either person’s body.

The number seven does not appear to have a mass or address.

A government can survive the replacement of every individual person who works for it.

A software program can be copied from one machine to another while remaining, in some sense, the same program.

These cases suggest that our everyday concept of “thing” may be too narrow.

Nature may contain objects, but perhaps it also contains patterns, processes, relations, and structures that cannot be reduced to the category of object without losing something important.

Physical Existence

The most familiar kind of existence is physical existence.

Physical things participate in causal processes.

They interact.

They occupy spacetime or are represented in physical theories as part of spacetime.

They can, at least in principle, influence observations.

This category includes obvious objects such as rocks and organisms, but modern physics complicates the picture.

What about an electromagnetic field?

It is not a solid object.

What about a photon?

What about spacetime curvature?

What about a quantum state?

What about a vacuum fluctuation?

Physics has repeatedly expanded the inventory of what we are willing to call physically real.

This should make us cautious.

Human intuition evolved around medium-sized objects moving slowly through ordinary environments.

Reality has no obligation to be composed only of things that resemble stones.

Objects and Processes

Consider a river.

Does the river exist?

Of course.

But what exactly is it?

The water molecules are constantly changing.

The shape changes.

The flow changes.

Sediment moves.

Banks erode.

Yet we speak of the same river across centuries.

This suggests that some entities are better understood as processes or persistent patterns rather than as collections of fixed material parts.

The same may be true of organisms.

The atoms in a human body are exchanged over time.

Cells die and are replaced.

Thoughts and memories change.

Yet personal identity persists strongly enough for ordinary life.

A flame provides an even clearer example.

A flame is not a particular collection of molecules.

It is an organized process maintained by continuous chemical reactions.

The matter passes through.

The pattern persists.

Perhaps existence sometimes belongs more naturally to patterns than to material components.

This idea will become important when we later discuss life, mind, software, and emergence.

Do Properties Exist?

A red apple exists.

But does redness exist?

A heavy stone exists.

Does mass exist independently of massive things?

A triangle exists on a page.

Does triangularity exist?

Philosophers have disagreed for centuries about universals—properties or forms that can be instantiated by many individual things.

A realist about universals may argue that properties have some genuine form of existence.

A nominalist may argue that only individual things exist and that general categories are names humans create.

The debate matters because science depends heavily on general properties and categories.

Electrons are treated as sharing properties.

Species group organisms.

Mathematics studies structures that appear repeatedly in different systems.

Are these similarities discovered in reality or imposed by our concepts?

The answer is not obvious.

Do Numbers Exist?

Few questions expose the problem of existence more sharply than mathematics.

Does the number three exist?

There are three stones on a table.

There can be three stars, three ideas, three sounds, or three computer files.

But the number three itself does not seem identical to any of these groups.

It cannot be weighed.

It does not decay.

It does not seem to occupy a location.

Yet mathematical truths appear strangely independent of us.

No civilization can decide that two prime numbers multiplied together will suddenly have a different product.

A mathematical platonist may say that numbers and mathematical structures exist abstractly.

A nominalist may argue that mathematics is a system of symbols and rules without independently existing abstract objects.

A structuralist may focus not on numbers as isolated objects but on positions within structures.

We do not need to settle this debate yet.

It is enough to notice that “existence” becomes ambiguous once abstract entities enter the discussion.

Do Possibilities Exist?

Imagine a glass on a table.

It could fall.

It could remain where it is.

Someone could move it.

A bird could fly through the window and knock it over.

Most of these possibilities will never happen.

Do they exist in any sense?

Ordinary language often treats possibilities as meaningful without treating them as actual objects.

But philosophy and physics both sometimes rely heavily on possible states.

Probability theory describes possible outcomes.

Quantum theory assigns amplitudes to possibilities.

Decision theory compares possible futures.

Modal logic formalizes statements about what is necessary and possible.

Some philosophers even defend forms of modal realism in which possible worlds are treated as real in a strong sense.

Others regard possibilities simply as ways the actual world could have been.

This distinction between actuality and possibility will return when we discuss laws, causality, quantum mechanics, and free will.

Social Existence

Consider money.

A banknote is physical.

But its monetary value is not simply a physical property of the paper.

If the relevant institutions and collective agreements disappeared, the paper would remain while the currency might cease to function as money.

The same is true of many social entities:

• citizenship,
• property,
• corporations,
• universities,
• borders,
• laws,
• offices,
• marriages,
• contracts.

These are not imaginary in the sense of being unreal.

They have consequences.

A legal decision can move people, money, and physical resources.

A border can determine where a person may live.

A corporation can own buildings.

Yet their existence depends on networks of beliefs, rules, records, and institutions.

This reveals another important lesson:

dependent existence is still existence.

Something does not need to exist independently of minds in order to be real in every meaningful sense.

Pain depends on nervous systems.

Language depends on users.

Money depends on institutions.

Software depends on physical implementations and interpretive conventions.

The fact that something depends on something else does not make it unreal.

Mental Existence

A thought exists differently from a chair.

You can point to the chair.

You cannot point to a thought in the same way.

Neuroscience may identify neural processes correlated with thinking, but the experience of thinking is available directly only to the thinker.

This creates one of philosophy’s oldest difficulties.

Are mental states identical to physical brain states?

Are they properties that emerge from them?

Are they descriptions at a different level?

Or does mind require a fundamentally different category of existence?

We will later examine these possibilities in detail.

For now, notice that denying the existence of mental states because they are not ordinary physical objects would be absurd.

Pain exists.

Doubt exists.

Fear exists.

Understanding exists.

Whatever their metaphysical status, they are part of reality as experienced by conscious beings.

Does a Hole Exist?

A hole is a useful test case.

A hole can be large or small.

It can be moved by moving the object containing it.

It can be counted.

One piece of cheese may have three holes and another five.

Yet a hole seems to be an absence of material rather than a material object.

How can an absence exist?

The puzzle is not trivial.

Physics also contains concepts involving absence, vacancy, empty regions, missing particles, and vacuum states.

The existence of a hole suggests that ontology may need to accommodate structured absences.

A missing tooth can cause pain.

An empty seat can matter.

Silence can be meaningful.

The absence of oxygen can kill.

The fact that something is an absence does not mean it has no role in explanations.

This will become especially important when we turn to nothingness.

Observer-Dependent and Observer-Independent Existence

Some things appear to exist whether or not anyone observes them.

A distant galaxy does not seem to need human attention in order to persist.

Other things depend partly on observers or communities.

A traffic signal is physically a device with colored lights, but the meaning of red as “stop” depends on a convention.

A sentence contains marks or sounds physically, but its linguistic meaning depends on language users.

A nation exists partly through institutions and collective recognition.

This suggests a distinction:

observer-independent reality does not depend on being interpreted by minds.

observer-dependent reality depends in some way on cognitive or social systems.

But the distinction must be handled carefully.

Observer-dependent does not mean arbitrary.

Once a society creates a legal system, individuals may face consequences whether or not they personally agree with it.

Once a language exists, individual speakers cannot freely assign words any meaning they like and still expect communication to work.

Emergent and social realities can constrain us.

Is Existence Binary?

We normally speak as if something either exists or does not.

But some cases resist clean classification.

Does a species exist?

Species are real enough for biology, but their boundaries can be fuzzy.

Does a cloud exist as one object when its edges continuously disperse into surrounding air?

When exactly does a developing organism become an individual organism?

When does a political movement become an institution?

When does a collection of interacting components become a system?

These questions may suggest that some categories are vague even if reality itself is not.

The uncertainty may lie in our concepts rather than in existence.

But there are also interpretations of quantum mechanics in which ordinary categories of definite actuality become difficult to apply before measurement.

We should therefore remain open to the possibility that the simple yes/no grammar of existence may not capture every level of reality equally well.

Existence and Explanation

Why does the question matter?

Because explanations depend on what we allow into our ontology.

Suppose a theory explains behavior using genes.

Then genes must count as real enough to play an explanatory role.

Suppose economics explains events using institutions, incentives, and markets.

These are not fundamental particles, but eliminating them from the vocabulary would make the explanation worse, not better.

Suppose computer science explains a failure in terms of an algorithm, data structure, or protocol.

Those concepts are not reducible in practice to a list of electron positions, even though the computation is physically implemented.

A useful ontology may therefore be layered.

At one level we speak of particles and fields.

At another, molecules.

At another, cells.

At another, organisms.

At another, minds.

At another, institutions.

The higher levels depend on lower levels but may still contain genuine patterns that support explanations and predictions.

This is one reason reductionism and emergence will later become central topics.

What Exists Without Us?

One of the deepest questions is what reality would contain if no conscious beings existed.

Stars presumably would still shine.

Planets would still orbit.

Radioactive atoms would still decay.

But would colors exist?

Would meanings exist?

Would money exist?

Would beauty exist?

Would mathematical truths exist?

Would information exist?

Would possibilities exist?

Some of these may depend on minds.

Others may not.

And some may force us to revise what we mean by dependence.

The distinction between a thing and the conditions required for recognizing that thing will matter repeatedly.

We should not assume that because humans need concepts to describe reality, reality depends on those concepts in order to exist.

At the same time, we should not assume that mind-dependent phenomena are unreal merely because minds are necessary for them.

A Working Principle

For this series, we will use a deliberately broad principle:

To exist is to belong to reality in a way that can make a difference to a coherent account of what there is and what happens.

This definition is not meant as a final metaphysical theorem.

It is a working tool.

It allows us to take physical objects seriously without pretending that only physical objects matter.

It allows us to discuss numbers without deciding immediately whether mathematical platonism is correct.

It allows us to discuss thoughts, institutions, information, and software without reducing every useful explanation to fundamental physics.

And it keeps the central question open:

What kinds of existence are fundamental, and what kinds depend on other things?

That question will follow us throughout Nature.

But now we can return to the question that first made existence seem strange.

If existence is not a single simple category, and if reality contains objects, processes, patterns, relations, minds, and perhaps abstract structures, then the deepest puzzle becomes even sharper:

Why is there any reality at all?

Why is there something rather than nothing?