How Do You Know If the Couch Fits? And Other Mysteries of Possibility

What If You Could Fit the Couch Through the Door?

You’re moving a couch into your room. You stop and stare. The door looks narrow. You picture the couch sliding through — and it seems impossible. But wait: how do you know it won’t fit? You haven’t actually tried. You just have a strong sense that it couldn’t be done. That feeling is a tiny piece of what philosophers call modal knowledge — knowledge of what is possible, impossible, or necessary.

Philosophers are interested in a special kind of modality, the one that isn’t merely about what you should do (like “you mustn’t steal”) or what you happen to know (“I know the cat is on the mat”). They study alethic modality, the objective ways things could or must be. Alethic modality splits into three flavors: logical, physical, and metaphysical.

• Logical possibility is the widest. Anything that doesn’t break the laws of logic is logically possible. A square circle? Not logically possible, because a shape can’t be both at once.
• Physical possibility is narrower — it obeys the laws of nature. A rock can fall off a cliff, but it can’t just float upward by itself.
• Metaphysical possibility sits in between. It cares about the deep nature of things, not just logic or actual physical law. Most philosophers think a world could have different laws of nature, so floating rocks are metaphysically possible even if physically impossible here. But a world where water isn’t H₂O is often thought to be metaphysically impossible, because being H₂O is part of what water is.

You can picture these as nested circles: the physically possible inside the metaphysically possible, inside the logically possible. A couch fitting through a door is physically (and therefore metaphysically and logically) possible. But could a couch turn into a river? That’s logically possible but physically and metaphysically impossible.

The big question that keeps philosophers busy is: How do we ever come to know what’s metaphysically possible or necessary? That’s called the Access Question. Strangely, we seem to have answers all the time. You know your table could break, even though it hasn’t. You know it must be either in the kitchen or not in the kitchen. But where does this knowledge come from: pure reasoning, experience, imagination, or something else entirely? The following views are three of the most powerful attempts to answer that.

Imagining Water Without Hydrogen: The Conceivability Way

One natural idea is that we know something is possible because we can conceive it — we can picture it in our minds in a coherent way. Two big names who developed this idea are Stephen Yablo (b. 1957) and David Chalmers (b. 1966) .

Yablo says that when you properly conceive a situation that would make a claim true, you have some evidence that the claim is possible. Suppose you imagine a world where the evening star (Hesperus) exists but the morning star (Phosphorus) does not. You might think, “Well, I can picture that, so it’s possible.” But we’ve known since the work of Saul Kripke (1940–2022) that this is a mistake. In our actual world, Hesperus is Phosphorus (both are the planet Venus), and if they are the same object, they cannot exist apart — it’s radically impossible. Yet many can still conceive it. So conceiving can mislead. Yablo responds that conceiving is like vision: your eyes can sometimes trick you, but they still give you reason to believe what you see. Conceivability gives prima facie justification — it holds until better reasoning or facts defeat it.

Chalmers goes further. He thinks a special kind of conceiving — ideal positive primary conceivability — actually entails a real kind of possibility. Let’s unpack that.

• Positive means you actively build a detailed scenario in your mind, not just fail to spot a contradiction (that would be negative conceivability).
• Ideal means you’re doing the best possible reasoning, free of mental limits and confusion.
• Primary conceivability works under the rule: “Imagine this scenario as if it were actual, knowing only what you can know a priori (independently of experience).”

Chalmers’s central example is water. Its chemical composition H₂O was discovered empirically. So the fact that water is H₂O is not a priori. When you conceive a world as actual without that empirical knowledge, you can coherently imagine that the clear drinkable liquid in lakes is not H₂O but some other stuff (call it XYZ). So “water is not H₂O” is primarily conceivable. According to Chalmers, that means it’s primarily possible — possible in a sense tied to how we first grasp the meaning of “water.”

But you cannot secondarily conceive it, because secondary conceivability works under a different rule: “Take for granted how things actually are, then imagine a world as counterfactual.” Since water actually is H₂O, trying to imagine a world where that very substance isn’t H₂O leads to contradiction. “Water is not H₂O” is secondarily impossible. So Chalmers can say: our ideal conceiving reliably tracks one kind of possibility, and when it fails to track the other kind, that’s because our concepts depend on hidden facts about the world.

Critics press a hard question: Why think that a purely a priori exercise of conceiving tells you anything about metaphysical possibility — the real, mind-independent way things could be — rather than merely about logic or concepts? That’s the Connection Question. And if we can only access primary possibility this way, how do we ever know about secondary metaphysical necessities? Chalmers’s route may be narrower than it first appears.

The Rock, the Bush, and “What If?”: Counterfactual Thinking

Timothy Williamson (b. 1955) offers a very different story. He thinks our capacity for modal knowledge is a by-product of something we do all day long: counterfactual reasoning — thinking about what would happen if things had been different.

Imagine you’re hiking in the mountains and see a rock slide into a bush. You wonder, “Where would it have ended if the bush hadn’t been there?” You picture the rock continuing down the slope and bouncing into the lake. By running that scenario in your imagination, using everything you know about rocks, gravity, and the landscape, you come to know a counterfactual: If the bush hadn’t been there, the rock would have ended in the lake.

Williamson argues that knowledge of metaphysical possibility and necessity is just a special case of this everyday skill. Why? Because there are tight logical equivalences. To say “It is necessary that gold is the element with atomic number 79” is equivalent to saying “If gold were not the element with atomic number 79, a contradiction would follow.” And to say “It is possible that the rock ended in the lake” is equivalent to saying “It is not the case that if the rock had ended in the lake, a contradiction would follow.”

So you can discover a necessary truth by supposing its negation in your imagination and checking whether you bump into a contradiction. Williamson thinks that we reliably do this by relying on our background knowledge: the basic laws of nature, causal relationships, and what he calls “constitutive facts” — deep truths about the identities of things, like gold’s atomic number. We keep some of this knowledge fixed while we develop the supposition.

But here’s the catch: how do we know which facts to hold fixed? In the rock example, you let go of the fact that the bush blocked the rock, but kept the fact that there’s a lake at the bottom. What if some other obstacle had been there? You used your sense of how the physical world works, but that sense may not always be reliable, especially when the question is not about rocks but about thorny metaphysical puzzles like whether a mind could exist without a body. This is the old problem of cotenability — what background beliefs count as “cotenable” with the imagined scenario? Without clear rules, we risk smuggling in assumptions that decide the answer before we even start imagining.

Essences: What Things Really Are

A recent wave of philosophers has placed essence at the center of modal knowledge. The idea draws on Kit Fine (b. 1946) , who argued that essence is not merely the same as necessity; rather, essences ground necessities.

If something is part of the essence of an object or kind — what it really is, its real definition — then necessarily that thing has that property. For example, part of the real definition of water is being H₂O. That is why water must be H₂O. So if you can figure out essences, you’ve unlocked a path to modal knowledge.

E.J. Lowe (1950–2014) thought that knowing an entity’s essence is simply knowing what it is. For some basic things, we can’t possibly not know their essence if we can think about them at all. Bob Hale (1952–2017) offered a two-part approach: we can know some essences a priori by understanding the meanings of words (for instance, grasping the word “rectangle” gives us the essence of being a rectangle), and we can know others a posteriori by discovering empirical facts, then plugging them into a deductive argument. Suppose you learn empirically that this metal is gold. A principle you know a priori — “If something is gold, it is essentially the element with atomic number 79” — allows you to conclude that it’s essential to that metal to have atomic number 79. Then from essence you deduce necessity: therefore, necessarily, that metal has atomic number 79.

Other essentialist accounts, like Antonella Mallozzi’s (contemporary) , stress that we often discover essences through science by finding the properties that powerfully explain why all samples of a kind share so many features. Atomic number explains why all gold is dense, shiny, and conducts electricity as it does. Once you spot such a superexplanatory property, you’ve found the essence — and you can then infer the modal claim.

Skeptics still ask: Are essences real, mind-independent facts? And even if they are, how do we come to know the bridge principle “If something is essentially F, then it is necessarily F” without already knowing what’s necessary? Answering these challenges is the project of the newest essentialist theories. But the appeal of the essentialist route is clear: it promises that our modal knowledge grows naturally out of understanding what things are — not from spooky access to invisible possible worlds.

Why Figuring Out Possibility Matters to You

You may never care whether a disembodied mind is metaphysically possible, but you care whether you can make the soccer team, whether you could have said something kinder in that argument, whether a bridge might collapse under a heavy truck. Every time you plan, regret, hope, or choose, you’re navigating possibilities and necessities.

This everyday modal thinking is the same kind of reasoning philosophers try to understand. If the counterfactual account is right, then when you replay in your head “If I had studied, I would have passed,” you’re doing miniature modal epistemology. If the essentialist picture is closer to the truth, then knowing what friendship or justice really is might tell you what you must and must not do to be a good friend. And if the limits of conceivability worry you, then you’ve already tasted the difference between what feels possible in your imagination and what is genuinely possible in the world — a distinction that matters every time we face big decisions.

The couch, the stars, the broken table — these homely examples open onto one of philosophy’s deepest puzzles: how creatures like us, stuck in the actual world, ever come to know what could and couldn’t be.

Think about it

1. If you can perfectly imagine a world where unicorns exist, does that prove unicorns are really possible in our world? What could make you change your mind?
2. Suppose a scientist tells you the deep nature of some new substance is a certain atomic structure. Does that automatically tell you what changes are impossible for that substance? Can you think of a case where knowing the essence doesn’t settle a possibility?
3. If a supercomputer could predict every decision you’ll ever make, would you still think you could have done otherwise? Why or why not?