What Is Real? The Strange Story of Substance

Here’s a weird thing philosophers noticed: when you look at a horse and a rock and your friend’s face, they’re obviously different things. But something about them is also the same—they all exist. They’re all something rather than nothing. What is that “something-ness”? What does it mean for anything to be real?

About 350 years ago, a group of philosophers called the rationalists tried to answer this question. They lived in different countries, argued with each other constantly, and disagreed about almost everything. But they shared one big idea: that reality has a deep structure you can figure out with reason alone, without relying much on your senses. And the key to that structure was something they called substance.

What Is a Substance?

The word “substance” sounds like it should mean something simple—like “stuff.” But for these philosophers, it had a very specific job. Substance was supposed to explain two things at once: why things are different from each other, and why things that are different can still be the same kind of thing.

Think about it this way. You and your friend are different people. You have different memories, different personalities, different bodies. But you’re both human. What makes you two separate individuals? And what makes you both humans?

A normal person might say “that’s just how the world is.” But philosophers wanted a deeper explanation. They wanted one concept that could handle both the difference and the sameness. That concept was supposed to be substance.

The ancient philosopher Aristotle had two different ideas about what substance was. In one of his books, he said substance is like a grammatical subject—the thing that has properties but isn’t itself a property of anything else. So “Bucephalus” (a famous horse) is a substance because you can say “Bucephalus is brown” but you can’t say “brown is Bucephalus.” In another book, though, Aristotle suggested that maybe substance is the form of a thing—what makes it the kind of thing it is. These two ideas pulled in opposite directions: one made each individual thing special, the other made all things of the same kind connected.

The rationalists took this tension and ran with it. Some of them pushed so hard toward the “sameness” side that they ended up saying there’s really only one substance in the whole universe. Others tried to hold onto individuality but struggled to explain how.

Descartes: The Father Who Started a Fight

René Descartes is often called the father of modern philosophy. He lived in the 1600s and was obsessed with finding something absolutely certain. He famously sat in a room, doubted everything he possibly could (including whether his body existed), and ended up with one solid thing: “I think, therefore I am.”

But Descartes had a weird side. He believed that all truth—even mathematical truth—depends on God’s will. God could have made 2+2=5 if He’d wanted to. This created a problem: if even logic isn’t fixed, how can you trust your own reasoning? Descartes thought the answer was that God is good and wouldn’t deceive us. But other philosophers immediately spotted a circular argument: you can’t use reason to prove God exists if you need God to prove reason works.

Descartes defined substance as “a thing that exists such that its existence does not depend on any other thing.” By that strict definition, only God is a substance. But he also allowed “created substances”—things that don’t depend on anything except God. These included minds and bodies. He thought mind and body were completely different kinds of substances: minds think but don’t take up space, bodies take up space but don’t think.

This created a famous problem. Descartes’ friend Princess Elisabeth of Bohemia wrote to him asking: if mind and body are completely different kinds of stuff, how do they interact? When you decide to move your hand, your mind (which isn’t physical) somehow makes your body move. How? Descartes gave a strange answer: there’s a special concept called “union” that you can only understand by NOT thinking too hard about it—just by living your normal life. Elisabeth was not satisfied. She suggested maybe part of the soul is extended (physical), which would make interaction possible. This was a remarkable suggestion from a 23-year-old princess in the 1600s, and it shows she was thinking circles around Descartes on this issue.

Spinoza: The Heretic Who Said Everything Is One

Baruch Spinoza was a Jewish philosopher living in Amsterdam who got kicked out of his community at 23 for his radical ideas about God. He took Descartes’ logic and pushed it further than anyone else.

Descartes had said that minds and bodies are different substances. Spinoza asked: if substance is defined as what doesn’t depend on anything else, how can there be two different substances that don’t depend on each other? He argued there can only be one substance, which he called “God, in other words, Nature.” Everything—you, the table, the horse, the stars—is just a mode (a temporary modification) of this one substance.

This is called monism: the view that reality is fundamentally one thing. It sounds weird, but Spinoza had a serious argument. If you have two different substances, they must be distinguished by their attributes (what kind they are) or their modes (how they’re modified). But modes depend on the substance they belong to, so they can’t tell you what’s a substance. So it must be the attributes that distinguish substances. But if substance is infinite and unlimited (which Spinoza thought it was), it must have all attributes. And if two substances can’t share an attribute, there can be only one.

The consequences were huge. If there’s only one substance, then there’s no free will in the usual sense—everything follows necessarily from the nature of that substance. Caesar crossing the Rubicon wasn’t a choice; it was as necessary as a logical proof. The universe is, in principle, completely intelligible to reason. There are no mysteries, just things we haven’t figured out yet.

Spinoza also solved Descartes’ mind-body problem. If mind and body are just the same thing conceived in two different ways, there’s no interaction problem. When your body runs, the corresponding ideas are in your mind. They’re not two things communicating; they’re one thing seen from two angles.

Malebranche: When Even Your Thoughts Belong to God

Nicolas Malebranche was a French priest who agreed with Descartes about many things but disagreed about a crucial point: he couldn’t accept that God’s truth depends on God’s will. He thought truth was higher than that.

Malebranche noticed something about knowledge. When you know that 2+2=4, you’re grasping something universal, necessary, and infinite. But your mind is particular, contingent, and finite. How can a finite mind grasp infinite truths? His answer: those truths aren’t in your mind; they’re in God. When you think, you’re actually consulting God’s mind. This is his doctrine of “vision in God”—we see all things through God’s ideas.

This might sound crazy, but it solves a real problem. If ideas were just inside your head, how could you be sure they match the outside world? For Malebranche, ideas are the blueprints God used to create the world, so they naturally match it.

He also had a radical theory of causation. For Malebranche, a true cause is something with a necessary connection to its effect. He argued that only God fits this description. When a billiard ball hits another and it moves, we think the first ball caused the second to move. But actually, it’s just an “occasion” for God to cause the motion. This is called occasionalism, and it means that nothing in the created world has any real power except God.

Leibniz: The Optimist with an Infinite Number of Souls

Gottfried Wilhelm Leibniz was the last great rationalist and possibly the most optimistic philosopher who ever lived. He thought we live in “the best of all possible worlds”—a claim that got famously mocked in Voltaire’s novel Candide.

Leibniz wanted to avoid both Spinoza’s monism (everything is one) and the occasionalism of Malebranche (only God does anything). His solution was to say there are an infinite number of simple substances called monads. Monads have no parts, no extension, no physical properties. They’re like souls—purely mental, active, perceiving things.

Here’s the clever part: monads are “windowless.” They don’t interact with each other at all. Each monad contains its entire history built in from creation, and its perceptions unfold according to its own internal nature. But God set up all the monads in a “preestablished harmony,” so they all perceive the same world from different perspectives. When you decide to raise your arm and your arm rises, it’s not because your mind caused your body to move. It’s because your mind-monad and your body’s dominant monad were both programmed to do matching things at exactly the same time—like two clocks that were set perfectly together and never touched again.

This let Leibniz keep individuality (unlike Spinoza) while avoiding the interaction problem (unlike Descartes). But it also meant that all knowledge is innate. You never really learn anything new from experience; you just become aware of what was already in your soul.

What Does This Have to Do with You?

These debates might seem like abstract nonsense. But think about what they’re really asking:

If you’re a different person than you were five years ago, what makes you still “you”? That’s the problem of sameness and difference.

If you decide to do something, how does your thought become a physical action? That’s the mind-body problem.

If you know something is true, how do you know you’re not wrong? That’s the problem of certainty.

These aren’t just philosopher problems. They’re problems about what it means to exist, to be a person, to know anything at all. The rationalists gave extreme answers—extreme enough that most people rejected them. But even in rejection, they forced everyone to think harder about what’s really real.

Nobody today believes in monads or vision in God. But the questions the rationalists asked are still alive. When scientists talk about consciousness, they’re grappling with the mind-body problem. When physicists talk about the fundamental nature of reality, they’re asking about substance. When anyone argues about free will, they’re echoing Spinoza.

The rationalists were wrong about a lot of things. But they were trying to take reason seriously—to see how far pure thinking could take you. Maybe that’s their real legacy: not the answers they gave, but the questions they refused to let go of.

Appendix

Key Terms

Term	What it does in the debate
Substance	The fundamental stuff of reality; supposed to explain both why things are individuals and why they’re connected
Mode	A temporary modification or way of being of a substance; not an independent thing itself
Monism	The view that reality is fundamentally one substance, not many
Monad	Leibniz’s term for simple, windowless substances that perceive the universe from unique perspectives
Occasionalism	The view that created things only occasion real causes; only God actually causes anything
Innate ideas	Ideas you’re born with, not learned from experience; rationalists believed these were the source of real knowledge
Preestablished harmony	Leibniz’s idea that all substances were set up at creation to perfectly coordinate without interacting

Key People

René Descartes (1596–1650): French philosopher who started modern philosophy by doubting everything; defined substance and created the mind-body problem
Princess Elisabeth of Bohemia (1618–1680): Sharp critic who pointed out the weakness in Descartes’ mind-body interaction; suggested the soul might be partly physical
Baruch Spinoza (1632–1677): Jewish philosopher expelled from his community for radical ideas; pushed monism to its extreme, claiming there’s only one substance
Nicolas Malebranche (1638–1715): French priest who said we see all things in God and only God has real causal power
Gottfried Wilhelm Leibniz (1646–1716): German philosopher and mathematician who believed this is the best of all possible worlds; invented monads and preestablished harmony

Things to Think About

If Spinoza is right that everything follows necessarily from the nature of the universe, can anyone ever be truly responsible for what they do? What would “responsibility” even mean in that world?
Leibniz’s monads are “windowless”—they don’t interact. Is that possible? Could you be a windowless monad, perceiving a world that doesn’t actually exist outside your perceptions?
Malebranche said only God causes things. But if that’s true, why do we bother making decisions or trying to change things? Does that make life meaningless, or could it mean something different?
Descartes argued that if two things can be conceived separately, they’re really distinct substances. Can you conceive of your mind without your body? Does that prove they’re different substances?

Where This Shows Up

Neuroscience and AI: the problem of how consciousness relates to the brain is still the mind-body problem Descartes started. People today argue about whether AI could be conscious using versions of the same arguments.
Free will debates: When people argue about whether free will exists or everything is determined, they’re working out consequences of arguments Spinoza and Leibniz already had.
“Why is there something rather than nothing?”: This question, which drives the whole rationalist project, is still asked by physicists and philosophers. Stephen Hawking famously said philosophy is dead, but he was still asking the rationalists’ question.
The simulation hypothesis: If you think you might be in a simulation, you’re asking a version of Malebranche’s question: is what I perceive really there, or am I just accessing a mind (or computer) that contains it?