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ecosystem terrarium with small plants
ecosystem terrarium with small plants

Philosopher: We Can’t Prove That We Aren’t Living in a Simulation

David Chalmers looks at the issues, step by step, in an excerpt from his new book, Reality+, and rules out proving that it is false

Philosopher David Chalmers, best known for the phrase “Hard Problem of consciousness” and the philosopher’s zombie thought experiment. tells us that we can’t actually prove that we are not living in a simulation: “You might think we have definitive evidence we’re not in a simulation. That’s impossible.”

The idea that we live in a simulation is basic to The Matrix films. People use the expressions red-pilled and blue-pilled every day now. The idea also underlies one of the explanations offered for why we don’t see extraterrestrials; according to the Planetarium Hypothesis, we are living in their “planetarium.”

It’s not just films and ET lore. Elon Musk has claimed to take seriously that we are aliens’ sims. So does Neil deGrasse Tyson (“Neil deGrasse Tyson says it’s ‘very likely’ the universe is a simulation”). Philosopher of science Nick Bostrom advanced the sim hypothesis in a seminal 2003 paper in Philosophical Quarterly. Former Astronomer Royal Martin Rees is sympathetic to the idea. Harvard astronomer Avi Loeb argued in Scientific American that we ourselves could advance in technology so as to create new universes.

If that all seems like moonshine, it ought to be easy to disprove. In an excerpt from his new book, Reality+: Virtual Worlds and the Problems of Philosophy (Norton, 2022) Chalmers outlines the difficulties:

Can you prove you’re not in a simulation? You might think you have definitive evidence that you’re not. I think that’s impossible, because any such evidence could be simulated.

Maybe you think the glorious forest around you proves that your world isn’t a simulation. But in principle, the forest could be simulated down to every last detail, and every last bit of light that reaches your eyes from the forest could be simulated, too. Your brain will react exactly as it would in the nonsimulated, ordinary world, so a simulated forest will look exactly like an ordinary one. Can you really prove that you aren’t seeing a simulated forest?

Maybe you think your darling cat could never be simulated. But cats are biological systems, and it seems likely that biological mechanisms can be simulated. With good enough technology, a simulation of your cat might be indistinguishable from the original. Do you really know that your cat isn’t a simulation?

Maybe you think the creative or loving behavior of the people around you could never be simulated. But what goes for cats goes for people.

David Chalmers, “Can We Prove the World Isn’t a Simulation?” at Nautilus (January 26, 2022)

He argues that even our bodies could be simulated so as to send the exact same messages to our brains and that our brains would not know the difference.

That said, Chalmers distinguishes between types of sim: Neo, in The Matrix, is a biosim, that is, part of him is biological and is not simulated:

In the ordinary spatial sense of “in,” Neo’s brain is not “in” the simulation. However, all his sensory inputs are coming from the simulation, and his outputs are going there, so he’s living in a simulation in the sense that matters. After he takes the red pill, his senses respond to the nonsimulated world, so he is no longer living in a simulation.”

David Chalmers, “Can We Prove the World Isn’t a Simulation?” at Nautilus (January 26, 2022)

But the “machine” characters Agent_Smith and the Oracle, are pure sims, artificial intelligences created for the sim. Presumably, the red pill wouldn’t help them as they have no alternative existence.

Generally, Chalmers thinks, a simulation would work best if it were global. Although local simulations have the advantage that they use less computing power, their limitations could be striking:

In the 1999 movie, The Thirteenth Floor, the simulators simulated only Southern California. When the protagonist tried to drive to Nevada, he encountered signs saying “Road closed.” He kept going, and the mountains morphed into thin green lines. That’s not a good way to design a convincing simulation. If a local simulation is completely local, it cannot properly simulate interaction with the rest of the world.

David Chalmers, “Can We Prove the World Isn’t a Simulation?” at Nautilus (January 26, 2022)
Clip from The Thirteenth Floor (1999). Language and violence warning.

Chalmers acknowledges a difficulty in simulating consciousness:

This issue— the issue of consciousness and whether a simulation could have it— is harder than the others. For now, we can set the issue of consciousness aside by focusing on impure simulations— that is, Matrix- style simulations in which you’re a biosim connected to the simulation. Biosims are not themselves simulated. They have ordinary biological brains which will presumably be conscious like ours. Whether you’re an ordinary person or a biosim whose brain is in the same state, things will look and feel the same to you. If so, we can never prove we’re not living in a simulation.

David Chalmers, “Can We Prove the World Isn’t a Simulation?” at Nautilus (January 26, 2022)

Even if we were told that we were in a simulation, he argues, that in itself could be a simulation.

So are we stuck never knowing? Well, there is a way out: Occam’s razor:

a scientific and philosophical rule that entities should not be multiplied unnecessarily which is interpreted as requiring that the simplest of competing theories be preferred to the more complex or that explanations of unknown phenomena be sought first in terms of known quantities – Merriam–Webster

William of Ockham

Or, at Conceptually: “Occam’s razor (also known as the ‘law of parsimony’) is a philosophical tool for ‘shaving off’ unlikely explanations. Essentially, when faced with competing explanations for the same phenomenon, the simplest is likely the correct one.”

William of Ockham (c. 1287–1347) did not invent the idea but he put it on the map.

The simplest explanation for the world we see around us is that it is reality. The fact that we can’t “prove” that we are not an extraterrestrial civilization’s terrarium does not mean that we need credit the idea that we are, for the same reasons as the fact that we cannot prove there are no elves means that we must credit their existence.

Except, of course, in the movies. That’s where imagination beats everyday reality — and, let’s face it, there we want it to.

You may also wish to read:

Will The Matrix Resurrections (drops December 22) break the mold? The culturally influential trilogy (control by evil aliens) enjoys a fascinating beginning — but a thud! ending. Can we really escape a world of illusions simply by following our most basic influences? If wisdom can’t help, why should instinct be the answer? (Gary Varner)


How can we be sure we are not just an ET’s simulation? A number of books and films are based on the Planetarium Hypothesis. Should we believe it? We make a faith-based decision that logic and evidence together are reasonable guides to what is true. Logical possibility alone does not make an idea true.

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Philosopher: We Can’t Prove That We Aren’t Living in a Simulation