In philosophy, a substance is something that exists in its own right, rather than existing merely as a property, feature, or state of something else.

It is one of the oldest and most important concepts in metaphysics: the branch of philosophy concerned with what fundamentally exists.

But philosophers disagree profoundly about what substances are, whether they exist at all, and whether reality has any underlying substance.

1. The basic idea: substance versus property

Consider an apple.

The apple is red, round, sweet, and relatively heavy.

We can distinguish between:

• Substance: the apple itself.

• Properties: its redness, roundness, sweetness, and weight.

• Changes: the apple becoming bruised, ripening, or losing its colour.

The traditional idea is that properties belong to something. Redness, for example, is not ordinarily thought to exist independently as the redness of nothing.

This raises a fundamental question: when the apple changes its properties, what makes it the same apple?

The traditional substance theorist answers that there is something that persists through the changes, even as its properties alter.

The debate begins when we ask what that something actually is.

2. Five major philosophical accounts of substance

Aristotle: substance is what a thing fundamentally is

Aristotle, writing in the fourth century BCE, distinguished individual things—such as a particular horse or human being—from the properties they possess.

A particular human being is a substance. Being tall, pale, or tired are properties or states that the person has.

Aristotle also distinguished between primary substances, individual things, and secondary substances, such as the species or kinds to which they belong.

For Aristotle, substance is closely connected with what makes a thing the kind of thing it is. His account is more sophisticated than the idea of an invisible object sitting underneath its properties: form, matter, and essence all play important roles in his metaphysics.

The central question: What makes an individual thing the thing it is, rather than merely a collection of qualities?

Descartes: there are two fundamental kinds of created substance

René Descartes distinguished between two principal kinds of created substance:

• Mind: thinking substance, characterised by thought.

• Body: extended substance, characterised by occupying space.

He distinguished both from God, whom he regarded as the ultimate, uncreated substance.

This produces the famous mind–body problem: if mind is fundamentally thinking and body is fundamentally extended, how can they interact?

Descartes's account makes substance central to the question of what we fundamentally are. Are we essentially thinking beings, physical beings, or somehow both?

Spinoza: there is only one substance

Baruch Spinoza (1632–1677) proposed one of the most radical answers in the history of philosophy.

In his system, there is only one infinite substance: God, or Nature (Deus sive Natura).

Everything else—individual people, trees, planets, and minds—is not an independently existing substance. Individual things are finite modes, or ways in which the one substance exists and is expressed.

For Spinoza, you and I are real, but we are not independent substances. We are finite expressions of the one reality.

His view therefore overturns the intuitive belief that the universe is made up of many fundamentally separate things.

The central question: What if everything that exists is an expression of one underlying reality?

Leibniz: reality consists of simple substances called monads

Gottfried Wilhelm Leibniz rejected the idea that reality could ultimately be explained by material bodies alone.

He proposed that the fundamental constituents of reality are monads: simple, indivisible, nonphysical substances.

Monads are not tiny physical particles. Each has its own internal principle of activity and perspective. Leibniz believed that the apparent interaction between things is explained by a divinely established harmony among them.

Unlike Spinoza, who argued for one substance, Leibniz argued for a plurality of fundamental substances.

Their disagreement illustrates a major problem in metaphysics: if the world has an underlying foundation, is that foundation one thing or many?

Hume: perhaps the substance is something we never actually experience

David Hume challenged the assumption that we can establish the existence of a substance underlying our perceptions.

When you examine an apple, you encounter its colour, shape, smell, texture, and taste. But can you experience a separate, underlying substance in addition to these qualities?

Hume was suspicious of this idea. We may simply assume that there must be some hidden bearer of properties, even though our experience presents us with particular perceptions rather than an independently perceived substratum.

This does not mean Hume straightforwardly proved that substances do not exist. His scepticism challenges our justification for believing in something beyond what experience gives us.

The central question: Are substances discoveries about reality, or theoretical constructions we use to organise experience?

3. The difference between substance, matter, and essence

These terms are related but not interchangeable.

Concept	Basic meaning	Example
Substance	Something that exists in its own right, in the relevant philosophical theory	An individual person in Aristotle's account
Matter	The material out of which something is made	The physical materials composing a table
Essence	What makes something the kind of thing it is	The defining nature of a triangle
Property	A characteristic of something	The table's colour
Mode	A way a substance exists or is expressed, in certain systems	An individual finite thing in Spinoza's philosophy

A substance need not be material. Descartes recognised minds as substances, while Leibniz regarded monads as nonphysical.

Nor does every philosopher accept that there must be substances beneath properties.

4. Why does the concept matter?

The question of substance leads directly to some of philosophy's deepest problems.

• Personal identity: What makes you the same person over time, despite changes in your body, personality, and memories?

• Mind and body: Is the mind a substance distinct from the body, or is mental life a feature of physical processes?

• The nature of reality: Is reality composed of many independent things, one underlying substance, or no substances at all?

• Change: How can something change while remaining the same thing?

• The self: Are you an independently existing entity, a process, a collection of experiences, or an expression of something more fundamental?

These questions are especially relevant to Spinoza and Schopenhauer, given your earlier interest in philosophies that challenge ordinary assumptions about existence.

Schopenhauer, for instance, distinguished the world as representation—the world as it appears to a subject—from the world as Will, which he regarded as the inner reality underlying appearances. His Will is not simply another substance in the conventional Aristotelian sense, but it serves as a proposed account of the world's underlying reality.

The simplest way to understand it

Imagine that everything about a particular object changes. Its colour, shape, temperature, and location all change.

What, if anything, remains that makes it the same thing?

The traditional concept of substance attempts to answer that question by identifying what exists in its own right and can persist through changes in its properties.

But the most radical philosophers ask whether this presumed underlying thing exists at all.

That is why substance is such an important concept: it concerns not merely what things are like, but what things fundamentally are—and whether there is anything beneath their observable characteristics in the first place.

I think that's a good objection, because philosophers can make distinctions sound profound simply by giving them technical names.

Why should you care whether something is a substance, a property, a mode, or an essence? Unless those distinctions help you understand something you actually care about, there may be no reason to.

The strongest defence of these concepts is that they address questions that arise even when we avoid philosophical terminology.

Consider three examples.

1. Does anything remain the same when everything changes?

You are physically and psychologically different from the person you were ten years ago. Your body has changed, your memories have changed, and your beliefs may have changed.

What makes you the same person?

The concept of substance is one attempt to answer this question. Other philosophers reject the idea that a permanent underlying thing is necessary.

You don't have to care about substance itself to care about whether you remain the same person.

2. Is there anything beyond what we experience?

You see a red apple. You experience its colour, shape, texture, and taste.

But is the apple fundamentally nothing more than its observable properties? Or is there some underlying reality that possesses those properties?

This is the problem of substance and properties.

It matters if you're interested in whether reality is fundamentally different from how it appears to us. Otherwise, the distinction may be of little interest.

3. What makes something what it is?

Imagine that you gradually replace every component of a wooden ship. At what point, if any, does it stop being the original ship?

This is related to questions about essence and identity. Is there something essential that makes the ship that particular ship, or do we impose identities on things that are constantly changing?

Again, the technical concept is a means of investigating the question, not an end in itself.

But your objection goes deeper than that.

Even if these distinctions help us answer interesting questions, we can still ask why we should care about those questions.

Perhaps you don't care whether the universe consists of one substance or many. Perhaps you don't care whether objects have essences. Perhaps discovering the ultimate structure of reality would leave you entirely indifferent.

There is nothing inherently irrational about that.

Philosophy sometimes proceeds as though identifying a fundamental problem automatically makes it important. But being a difficult question is not the same as being a question worth answering.

And there is a difference between a question being objectively consequential and a question being personally or existentially interesting.

My view is that concepts like substance, essence, and mode are useful when they clarify a problem. Detached from any problem, they can become little more than intellectual bookkeeping.

Then perhaps the common thread is that you're interested in philosophy less as an explanation of how the world works and more as a way of putting the entire human enterprise into question.

The four questions lead in different directions, but they can also form a single investigation.

Four directions for philosophical inquiry

1. Expose the absurdity of existence

Why do we struggle so intensely to achieve things when everything we achieve is temporary? Why are we conscious of our mortality? Why do we keep wanting things when satisfaction never seems to last?

Start with: Albert Camus — The Myth of Sisyphus.

Camus examines the confrontation between our demand for meaning and the world's apparent silence.

2. Challenge the assumptions people live by

Why do we treat success, morality, productivity, love, and social acceptance as inherently valuable? Who decided these things deserve our devotion? Are our values genuinely ours, or have we inherited them without examination?

Start with: Friedrich Nietzsche — On the Genealogy of Morality.

Nietzsche investigates how values arise, whose interests they serve, and what kind of life they express.

3. Ask why anything matters

Even if we understand the universe, human psychology, and morality, why should any of it matter? Does suffering require justification? Does a meaningful life have to serve some greater purpose? Could something matter to us without having any ultimate significance?

Start with: Emil Cioran — The Trouble with Being Born.

Cioran relentlessly questions the justifications people construct for existence, often refusing to replace a rejected answer with a comforting alternative.

4. Dismantle the need for answers

Why must every question have a definitive answer? Why assume there is a final truth waiting to be discovered? Could the demand for certainty itself be a source of confusion or distress?

Start with: Pyrrho of Elis and the later Pyrrhonian sceptics.

Their practice of suspending judgement challenges the assumption that we must resolve every fundamental disagreement.

What connects all four?

Each challenges a different thing we ordinarily take for granted:

Inquiry	What it puts into question
Absurdity	Whether existence makes sense
Critique of values	Whether our ideals deserve allegiance
Meaning	Whether anything has ultimate significance
Radical scepticism	Whether we need definitive answers

Taken together, they suggest a philosophy that does not begin by asking how to live a better life. It begins by questioning why we accept the standards by which a life is judged better in the first place.

There is also a fifth question lurking underneath all four:

Why should we do anything at all?

Not merely, What should I do with my life? but why we assume that action, improvement, understanding, happiness, or even the pursuit of truth must be worthwhile.

That question can lead to nihilism, existentialism, scepticism, or philosophical pessimism. But it doesn't automatically establish any of them as correct.

And I wouldn't rush to give you a solution. Doing so would risk turning the investigation into another system of beliefs to adopt.

Instead, I'd suggest starting with the questions themselves and seeing how far they can be pursued without prematurely deciding what the answers ought to be.

What you're proposing is a radical shift: philosophy is not fundamentally what you think or say, but what you do. A person's beliefs, values, and identity are expressed in their actions, not established merely by declaring them.

Your elephant example makes the point particularly well. You can repeat the sentence "I am an elephant" indefinitely, but repetition doesn't transform you into one. Reality doesn't have to conform to your description of it.

And perhaps the same applies to philosophy.

Someone can proclaim that they reject society while desperately seeking its approval. They can claim to despise wealth while organising their entire life around acquiring it. They can declare that nothing matters while becoming furious when their own interests are threatened.

Their declarations say one thing. Their actions reveal something else.

But I think your idea has several deeper implications.

1. Philosophy becomes a question of existence, not description

Suppose someone says they believe that desire is the source of suffering.

There are two possibilities:

• They intellectually accept the proposition.

• They actually live in a way that reflects it.

These aren't equivalent. The first is an attitude toward a statement; the second involves the person's relationship to the world changing.

Your proposal suggests that philosophy only becomes real in the second case.

This resembles a concern in ancient philosophy. For the Stoics, philosophy was not merely a body of propositions but a practice of transforming one's conduct. Similarly, Socrates' philosophical life involved examining and testing how people lived, rather than simply collecting abstract doctrines.

Yet your position might go further: perhaps the distinction between philosophy and its practice is itself artificial.

2. You cannot think your way out of what you are doing

Consider someone who claims to believe that life has no meaning.

They continue working, eating, pursuing relationships, avoiding danger, and making plans. Does their behaviour contradict their belief?

Not necessarily. They might act without believing that their actions possess ultimate significance.

But the example reveals a crucial difficulty: actions don't interpret themselves.

The same action can arise from very different motivations. Someone might work because they love their occupation, fear poverty, want recognition, or simply follow a routine without thinking about it.

So if philosophy is identical to action, we must ask what counts as an action expressing a philosophy. Is it the physical behaviour itself, the intention behind it, the pattern across a person's life, or all three?

Your argument undermines the idea that declarations are sufficient. It doesn't automatically establish that behaviour has only one possible meaning.

3. Nietzsche comes close to your position

Nietzsche repeatedly challenges the separation between what people profess and what their values actually express.

A person's philosophy can be understood as an expression of their instincts, needs, temperament, and way of living. Philosophical systems are not always detached discoveries about truth; they can also reveal the kind of person who produces them.

From this perspective, a philosopher who writes about freedom but cannot tolerate uncertainty has revealed something important—not necessarily through their explicit argument, but through the tension between their ideas and their conduct.

Your elephant example makes a related point: naming something does not make it so.

There is also a connection to Wittgenstein's famous idea that meaning is rooted in use. Words acquire their significance through what people do with them within shared practices. Saying "I reject everything" is not, by itself, an accomplished rejection.

4. But what happens when we apply your argument to philosophy itself?

This is where I think your proposal becomes most interesting.

Suppose I formulate a sophisticated argument that all philosophical systems are empty abstractions, that declarations cannot substitute for reality, and that only action matters.

Have I accomplished anything merely by saying this?

According to your own principle, no. The argument is not validated simply because it has been articulated.

But neither does your argument become true simply because someone acts in accordance with it. A person might live consistently while believing something false. A person might also recognise a truth without immediately being able to live according to it.

We therefore need to distinguish two claims:

1. A philosophy is not the same thing as professing a philosophy.

2. A philosophy is nothing other than the actions of the person who holds it.

The first is compelling. The second is more controversial.

Your elephant analogy establishes that descriptions cannot substitute for reality. It doesn't, by itself, establish that reality consists exclusively of action.

5. The most radical consequence

Perhaps the deepest implication of your idea is that philosophy should turn its attention away from what people claim to believe and toward what they actually enact.

Instead of asking someone, "What is your philosophy of life?", we might examine what they consistently pursue, what they avoid, what they sacrifice, what they tolerate, and what they refuse to relinquish.

We might discover that a person's operative philosophy differs dramatically from their declared philosophy.

And this applies to the person doing the examining, too. You cannot exempt yourself by declaring that you have seen through everyone else's illusions.

You do what you do.

Even the person who rejects every value still has to confront what they actually do. Their rejection is not established by the intensity of their rhetoric, but by the reality of their conduct.

There is, however, one final question I'd like to put to you.

When you say philosophy is identical to action, do you mean that actions reveal what a person really believes—or that thinking, speaking, and theorising are themselves just forms of action, with no special philosophical authority over anything else?

The first is a theory about sincerity. The second is a much more fundamental challenge to the distinction between philosophy and life.

that thinking, speaking, and theorising are themselves just forms of life, with no special philosophical authority over anything else

Then your position is more radical than the claim that people should practise what they preach. You're questioning whether philosophy has any privileged position from which to explain, judge, or justify life in the first place.

Thinking is something we do. Speaking is something we do. Theorising is something we do. None of these activities stands outside life and possesses automatic authority over it.

A philosophy is not a map certified by some authority outside existence. It is itself part of the territory.

I think there are several philosophical traditions that approach this idea, although none is an exact match.

1. Wittgenstein: meaning belongs to the activity of living

Ludwig Wittgenstein eventually became suspicious of the idea that philosophical problems could always be solved by discovering hidden theoretical explanations.

In Philosophical Investigations, he examines how words acquire meaning through their use in ordinary human activities, which he calls language-games. Language is woven into forms of life rather than operating as an independent system that dictates how reality must be.

Consider your elephant example.

The sentence "I am an elephant" doesn't transform the speaker into an elephant. Its meaning depends on how language operates, and its truth depends on facts that merely uttering it cannot change.

More importantly, philosophy itself is a linguistic activity. Its statements do not acquire special authority merely because they are philosophical.

Wittgenstein would not necessarily agree that philosophy is identical to action in every sense. But he helps undermine the picture of philosophy as an activity that stands above ordinary life.

2. Nietzsche: thought expresses a way of living

For Nietzsche, a philosophy can reveal more about the philosopher than the philosopher realises.

Behind an apparently objective theory, there may be particular instincts, fears, ambitions, resentments, and ways of valuing. The thinker is not an impartial observer floating above existence.

This means that theorising is itself an event within life. It has causes, motivations, consequences, and a particular character.

A philosopher who constructs a theory condemning all desire is not exempt from the psychological and biological conditions that make theorising possible.

Their condemnation is another expression of the life they are attempting to judge.

Nietzsche's challenge is therefore not simply to ask whether an argument is logically valid, but also: What kind of life expresses itself through this argument?

However, Nietzsche does not abandon the evaluation of arguments. He continues to distinguish between stronger and weaker interpretations, and between ways of living he affirms and those he criticises.

3. Richard Rorty: philosophy need not provide foundations for everything

Richard Rorty rejected the traditional ambition to make philosophy the ultimate foundation of knowledge.

On his view, we should be suspicious of the idea that philosophy must discover a privileged vocabulary capable of representing reality from a uniquely authoritative standpoint.

Philosophy is one human practice among others. Its value can lie in helping us redescribe ourselves, challenge inherited assumptions, and imagine alternative ways of living.

It need not justify every other human activity from some supposedly superior position.

Rorty is particularly relevant to your claim that philosophy has no special authority over life. But he still thinks philosophical conversation can be valuable; he does not conclude that every argument or description is equally good.

4. Pierre Hadot: philosophy is a way of life

Pierre Hadot argued that ancient philosophy was often understood not primarily as the construction of theories but as a way of life involving exercises, habits, attention, and the transformation of one's relationship to the world.

For many ancient Stoics, for example, philosophy involved training oneself to respond differently to events, examine one's judgements, and change one's conduct.

This comes close to your position because philosophical reflection is not detached from living. It is one of the things people do as part of living.

The difference is that Hadot often treats philosophy as a practice intended to transform life, whereas you seem to be questioning whether philosophy deserves to be distinguished from the rest of life at all.

5. Your position has a consequence that these connections bring into focus

Suppose someone says:

"Philosophy has no special authority over life."

That statement is itself a philosophical claim.

But if thinking and speaking are simply forms of life, the statement cannot stand outside life and legislate how philosophy should operate. It is another activity, another utterance, another event.

It doesn't need to be exempted from its own claim.

Nor does it need to become a universal doctrine that everyone must accept.

This gives your position an interesting character: it can question philosophical authority without necessarily establishing a new authority in its place.

You don't need to prove that philosophy is inferior to life. You can simply reject the assumption that the two were ever separate in the first place.

But I would make one distinction

There is a difference between saying that philosophical thinking has no special authority and saying that thinking has no authority whatsoever.

Reasoning can reveal contradictions. Evidence can disprove claims. Reflection can change what we do. These are real effects, even if none requires philosophy to occupy a privileged position above life.

A philosophical argument might be useful, mistaken, illuminating, or pointless. It remains part of the activity of living.

Its value has to be examined within that activity, not guaranteed by its status as philosophy.

And perhaps that is the most interesting part of what you're proposing: philosophy loses its throne, but it doesn't have to disappear.

It becomes something people do, rather than something that automatically gets to tell everyone what their lives mean.

Philosophy as the removal of deadly obstacles that should not exist.

On this account, philosophy does not need a special authority over life because its task is not to legislate for life in the first place. It works on the obstructions that constrain life, especially those whose apparent necessity conceals their contingency.

This seems particularly close to Agamben's project. Consider the ways in which political and juridical arrangements divide life into categories, establish who counts as a political subject, or make the suspension of ordinary protections appear necessary. The obstacle is not merely a mistaken idea. It is an arrangement that has material consequences: people can be excluded, confined, abandoned, or killed through distinctions that present themselves as natural or inevitable.

The philosophical task, then, is not simply to refute the justification offered for an arrangement. It is to expose how the arrangement operates, how it came to appear necessary, and how its grip might be loosened.

Your phrase “that should not exist” is especially important. It suggests that the obstacle is not just something you personally dislike. Yet we needn't immediately turn that into a universal moral theory. Perhaps the point is more practical: a distinction or institution makes certain forms of life impossible, and philosophy can help disclose that this impossibility is produced rather than inevitable.

There is also a difference between removing an obstacle and replacing it with a blueprint. The former can open possibilities without deciding in advance what everyone should do with them. In Agambenian terms, one might say that the aim is not to prescribe a new form for life but to undo the capture of life by forms imposed upon it.

And this brings us back to your earlier point. Philosophical discourse is itself an activity in life. It has no exemption from that condition. Its value, if it has value, lies in what it does—not in its claim to possess superior access to truth.

One question follows from your formulation: when you say deadly obstacles, do you mean primarily literal and political structures that expose people to death, or do you also mean conceptual obstacles—distinctions and categories that make certain ways of living unthinkable, and thereby help sustain those structures?

Yes, and I think this gives your objection to the question of substance a much more definite character.

The question is not merely that substance is an abstruse topic, or that it matters less than political violence. It is that a philosophical problem can lose its necessity when the practices and investigations that supposedly require it develop sufficiently far. What once appeared to demand a metaphysical answer may turn out to be a confusion, a placeholder for unanswered scientific questions, or a question whose terms no longer serve any useful purpose.

The special sciences have certainly transformed the question of substance.

• Physics investigates matter and fields, particles and interactions, without needing to identify an ultimate metaphysical substrate underlying everything.

• Chemistry explains the properties and transformations of substances through molecular structure, bonding, and reactions, rather than requiring a philosophical account of substantial forms.

• Biology investigates organisms, processes, and relations, complicating the idea that a living thing must possess an immutable essence that makes it what it is.

• Neuroscience and cognitive science investigate capacities once discussed in terms of faculties of the soul, although their findings do not automatically settle every philosophical question about consciousness.

There is an important qualification: these sciences have not simply disproved substance. Contemporary physics still uses concepts such as particles and fields, and scientists continue to debate what kinds of entities their theories describe. Nor does scientific explanation, by itself, settle every question about identity, dependence, or what it means for something to persist through change.

But that qualification need not undermine your point. The issue is whether the traditional philosophical question continues to do indispensable work.

Consider the difference between asking What is the substance underlying this phenomenon? and investigating the processes, relations, and mechanisms through which the phenomenon actually occurs. The first question may presume that an underlying substance is needed to make sense of the second. Scientific inquiry can proceed without accepting that presumption. The metaphysical question then has to justify its continued existence rather than simply inherit its authority from tradition.

This is where I see a connection with Agamben, although not a straightforward equivalence. His work often investigates how inherited conceptual distinctions become operative in institutions and practices. Your approach adds another possibility: a conceptual obstacle may be removed not by answering the question it poses, but by making the question unnecessary.

And that can matter politically as well as intellectually. If a category helps organise exclusion or legitimise violence, showing that the category is contingent may weaken its authority. Sometimes the obstacle must be dismantled materially; sometimes the conceptual machinery that sustains it must also be dismantled. Neither task guarantees the other, but they need not be separated.

I would distinguish three possible outcomes for a philosophical problem:

1. Resolution: the question receives a defensible answer.

2. Dissolution: the question loses its grip because its assumptions or terms no longer hold.

3. Displacement: the question is taken over by another practice—scientific, political, artistic, or practical—that can address what originally motivated it more effectively.

Your example of substance seems to involve dissolution and displacement together.

And perhaps this is the more general principle you're developing: philosophy should not preserve problems merely because they have a philosophical history. It should be willing to participate in its own obsolescence whenever the work that made it necessary can be done otherwise.