In debates (or even just discussions) people sometimes ask how we know the physical world is really there. A variation asks how we know that what we perceive as the real world is the same as what other people perceive. (One example of this is the inverted spectrum.)

The most accurate answer is: We don’t. Not for sure, anyway. There is at least one assumption built in, but it’s one we have to make to escape our own minds. According to ancient philosophical tradition, the only fact we know for sure is that we ourselves exist. (Although I think there’s an argument to be made about a priori knowledge.)

But, as with the excluded middle, accepting reality as an axiom seems almost necessary if we’re to move forward in any useful way.

The excluded middle is the philosophical axiom that if something is not true, it must be false (and if it’s not false, it must be true). There is no middle; propositions are always either false or true.

In life, there can be situations complex enough to have no answer or even to be equally true and false, but in mathematical logic the excluded middle allows powerful proofs of theorems. A basic tool is to assume something is true, establish a contradiction, and thus prove your assumption false.

The Turing Halting problem and Cantor’s uncountable infinities both depend on the excluded middle (as do many others).

It’s hard to come up with why it shouldn’t be an accepted axiom. Propositions are either true or false, never both, never null. Even most messy real life situations, when you boil them down, the parts, at least, are either true or false.


Something similar happens in accepting the external world as axiomatic.

We start with Descartes famous “Cogito ergo sum,” which gives us a fact: We exist (because we think).

I believe there are other facts we can apprehend with no notion of any world outside our minds. For example, the passage of time and the concepts of mathematics. (I’ve argued before that math is inevitable.)

When we consider the input of our senses, our sight and sense of touch especially, we perceive an apparent external world. But this world comes to us through our senses as impulses in our nerves (and these impulses do not particularly look like the physical world; they’re just signals).

The “brain in a jar” question (or the “virtual reality” question) are essentially impossible to answer with 100% certainty.

But that doesn’t mean we can’t answer them with a high degree of certainty.

High enough to take on faith (least we sink into solipsism).


The key lies in accepting two appearances as reflecting true reality:

  1. The external world exists and appears to us.
  2. Others like us exist and report their view of reality.

The first we test through our empirical experiences. If we move an object, it stays moved. If we break a glass, it stays broken. Hot things cool. Dropped things always fall. Even the exceptions prove the rules (because even the exceptions operate according to rules).

We find, through our empirical experiences, that an apparently consistent external reality exists and it follows laws we can discover. (What’s more, those laws can be expressed very nicely using our a priori mathematics! We’re not exactly sure why this is true.)

We humans are notoriously known to be great at “pattern matching,” and that’s because the external world is filled with consistent patterns that follow consistent laws. Science is all about finding those patterns and laws.

The next step involves special parts of that external world: active agents who, if we believe what they say, are very much like us. They seem to think the same, and they report existing in apparently the same world we do.

They report finding the same laws and patterns.

By all indications, a consistent external reality exists, and it is populated with others like us who confirm that reality.

It takes some serious skepticism to consider otherwise!


It’s not proof, not for certain, but it is what we might call an overwhelming preponderance of evidence.

Further, over time, a consensus — a common view — emerges. As that view increasingly survives any challenges, we begin to trust it as a conditional true fact (conditional on some putative future fact that falsifies it).

This is exactly the process of science: Observing reality, thinking about it, building a model that we think explains it, and then testing that model to see if it fails.

Or as a bumper sticker: See, Think, Theorize, Test.

We’ve each been doing this since we were born: Seeing (touching, smelling, hearing, tasting) the world around us and building a model of that world in our minds.

Ideally, when confronted with contrary facts, we adjust our model to better match all the facts. Science is so successful because this self-correction is fundamental to its process.

It should be obvious that having a mental model of the world that doesn’t match the facts can’t lead to long-term success. Reality always wins in the end.

(Trying to change the world to better match what’s in your head is a different thing entirely, and people have fallen on both sides of trying that.)

In fact, my definition of sanity is primarily based on how well your mental model of reality matches the real, physical world. (Which, after thousands of years of human empiricism and communication, with billions participating in the Great Experiment, yes, I do believe is pretty much what it appears to be.)

People who deny physical reality aren’t sane to me.

And I don’t quite know what to make of people who turn their backs on physical reality. To me it seems willfully insane. (Certainly willfully stupid.)


Bottom line: Firstly, we can’t ever really 100% prove anything beyond Cogito ego sum, so we can only accept external reality (and others like us in it) as axioms. We can either accept them or not. If we don’t, we’re stuck, alone in our own minds.

Given the strict consistency of our empirical experiences, in particular the appearance of unbreakable physical rules and patterns, it seems reasonable to accept these experiences as indicative of a real, lawful, external world.

Part of that external world appears as active agents claiming parity with us; their reported experiences match ours exactly. It seems reasonable to accept these reports as true, and therefore their agents as being what they appear to be, other people like us.

If this is all happening in my mind, I have to say I’m impressed by my imagination (and a bit depressed by how dark it’s gotten)! I’m also impressed how I’ve doled out apparently learning new things constantly for 60-some years.

(I’ve hidden understanding quaternions from myself for decades. Maybe I’ll soon let me understand tensors!)

Pretty silly, right? Even if I were a brain in a jar, or in a virtual reality, there still seems an external reality that unfolds over time. Someone, or something, is running the experiment and providing new inputs.

We’re so far from understanding what’s really going on. (Some believe our minds aren’t capable of it.)

It’s interesting that the ontology behind General Relativity seems fairly clear and simple: mass (or energy, same thing) warps spacetime. Why? Dunno, it just does. But it kinda makes sense: heaviness bends things.

Compare that to Quantum Field Theory, our other great theory of reality, which has wildly diverging putative ontologies (Copenhagen, Many-Worlds, Pilot-Wave, etc), most of which seem to contain bits of magic. No one agrees on what’s really going on.

We just accept quantum entanglement — Einstein’s “spooky action at a distance” — as part of the package!

But I digress.

Once again: Bottom line: We dunno for sure, but it’s a damn good bet.

Reality is real.

About Wyrd Smythe

The canonical fool on the hill watching the sunset and the rotation of the planet and thinking what he imagines are large thoughts. View all posts by Wyrd Smythe

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