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Things We Can Doubt
1. It has been several years now since I first realized something unsettling. From the time I was young, I had been accepting all kinds of opinions as true without ever really checking whether they were. And if the foundations were wrong, then everything I built on top of them was probably wrong too.
So, I made a decision. At some point in my life, I was going to have to tear the whole thing down and start over. Not just fix a few mistakes here and there—that would be like patching a rotten wall instead of rebuilding it. I needed to go all the way down to the foundation and start fresh, if I ever wanted to build something I could actually trust.
But that’s a big project. So, I waited. I kept waiting until I was old enough and mature enough that I couldn’t keep telling myself I’d be better prepared later. That time is now. Today, I find myself with a quiet mind, free from distractions, with the time and the peace to actually do this. (Descartes was living in peaceful retirement when he wrote this—deliberately setting aside time for serious philosophical work.)
So, today I begin. I am going to systematically set aside every opinion I have ever held. All of it goes.
2. Now, I want to be clear about something. I’m not going to try to prove that all my former opinions are completely false. That might actually be impossible—and it isn’t necessary anyway.
Here’s why. If I find even one good reason to doubt something, that’s enough to set it aside. I don’t need to prove it’s wrong. I just need to show it isn’t completely certain. (Think of it this way: if you’re not sure whether the milk is bad, you don’t drink it. You don’t need to prove it’s definitely sour.)
And I’m not going to go through every single opinion one by one either. That would take forever. Instead, I’m going to do something much more efficient. I’m going to attack the foundations. If the foundations crumble, everything built on top of them crumbles too. So, the question is: what are my most basic beliefs built on?
3. Everything I have ever believed with complete confidence came to me through my senses—through seeing, hearing, touching, tasting, smelling. My senses have always felt like my most reliable guide to what's real.
But here’s the problem. My senses have deceived me before. And if something has deceived you even once, it’s not wise to trust it completely.
4. But wait—someone might object here. Sure, our senses can mislead us about small things or distant things. Maybe I misjudge the size of something far away or miss a detail too tiny to see clearly. But what about the obvious stuff? Surely I can’t doubt that I am sitting here right now, by the fire, wearing my dressing gown, holding this piece of paper in my hands. Some things (“presentations”) are just too immediate and obvious to doubt.
And honestly, that’s a fair point. How could I seriously deny that I have hands? That I have a body? That I’m sitting exactly where I think I’m sitting? Only a madman would doubt something that obvious. (Descartes is thinking here of people who suffer from serious delusions.) I’m thinking of people who sincerely believe they are kings when they are penniless. Perhaps they think they are dressed in royal robes when they have no clothes at all or that their bodies are made of glass or clay. Or maybe they think they are a squash. (These weren’t just hypothetical examples to Descartes. Such delusions were documented in his day.)
To doubt something as obvious as my own hands would put me in the same category as those poor souls.
5. But here’s the thing. Even if doubting my own hands would make me sound like a madman, don’t I sleep? I am a human being, and human beings dream. Think about it. When I’m dreaming, I often experience things just as vividly as I do right now. Maybe I’m sitting by the fire, wearing my clothes, holding this paper. Dreams can feel completely real while they’re happening. How many times have I dreamed I was sitting exactly where I'm sitting now, only to wake up and find myself lying in bed?
Right now, in this moment, everything feels obviously real. My eyes are open. I’m moving my head. I’m consciously extending my hand and watching it move. Surely this is different from a dream. Dreams aren’t this clear and vivid, are they?
But wait. I’ve thought that before. And I was dreaming.
The more carefully I think about this, the more unsettling it becomes. I cannot find a single reliable test—not one—that would definitively prove I’m awake right now rather than dreaming. And that realization is genuinely shocking. In fact, the more I think about it, the more I start to wonder... Am I dreaming right now?
6. OK, let’s go with it. Suppose we’re actually dreaming right now. Let’s suppose that opening my eyes, moving my head, extending my hands— it’s all just part of the dream. Let’s even imagine that I don’t even really have a body or hands at all.
Even if that’s all true, here’s the interesting thing. Dreams have to be made of something. When we dream, we dream of things—faces, hands, rooms, fire. Those dream images had to come from somewhere.
They’re like paintings. And here’s the thing about paintings. Even the most imaginative painting—like a siren, a dragon, or a creature that has never existed—is still made up of parts taken from real things. A painter can combine a woman’s face with a fish’s tail, but she can’t invent something from absolutely nothing. Even if every element is rearranged into something new and fantastical, the colors themselves are real.
(Think of it this way. Even the most bizarre dream you’ve ever had was made up of things you’ve actually seen or experienced. Your mind doesn’t create from nothing. It recombines things you’ve experienced.)
So even if everything in my dreams is imaginary and unreal, the basic building blocks must be real. The simple, universal elements that dreams are constructed from are real, things like eyes, hands, and bodies.
And even more basically than that, there are things so fundamental that they are the assumptions that make all these other images in our minds possible. These are simple, universal concepts like shape, size, quantity, and the fact that things exist at all in space and time. This is true whether we are talking about real or imaginary things, whether we are awake or dreaming.
7. So, what kinds of concepts belong to this most basic level of reality? These are ideas like physical existence in general. The idea that things take up space, have shape and size. We can count them. We can locate them somewhere in space and time.
8. This leads to a surprising conclusion about the sciences. Take sciences like physics, astronomy, or medicine. They study complex, “composite” things (things made up of parts, not “simple” things). Our train of thought puts them on shaky ground. They depend on assumptions about the physical world that we’ve just called into question. If we are dreaming, we can’t fully trust what our senses tell us about the physical world. And those sciences overwhelmingly depend on our senses.
But subjects (Descartes calls them “sciences” too, as the people of his day did) that deal only with these simple, universal concepts are on much firmer ground. These are sciences like arithmetic and geometry. In fact, they might be the most certain knowledge we have!
Here’s why. Whether I’m awake or dreaming, two and three still make five. Whether I’m awake or dreaming, a square still has four sides. These truths don’t depend on whether the physical world is real or not. They don’t depend on our senses at all. They just are true, no matter what.
9. But wait. There’s something I’ve believed for a long time that complicates everything we just said about mathematics.
I believe in God. And not just any god. I believe in an all-powerful God who created me and everything about me. And if that’s true, it raises an uncomfortable question. What if God arranged things so that there is no earth, no sky, nothing that takes up space, no shapes or sizes or locations? What if he has only made me think those things are real? What if my entire experience of reality is just God feeding false perceptions directly into my mind?
And here’s where it gets even more unsettling. I sometimes notice that other people are completely confident they know something—when they’re completely wrong! What if that happens to me with math? What if every time I add two and three and get five, I’m actually wrong? What if I’m actually wrong to think a square has four sides? What if God is allowing me to be deceived even about something that simple?
Here’s where someone might object, “But God is supposed to be supremely good. Surely a good God wouldn’t deceive me like that?”
But think about it. If God’s goodness means he wouldn’t let me be deceived at all, then why does he allow me to get things wrong sometimes? Yet, clearly, he does.
Apparently, God’s goodness doesn’t protect me from being deceived or wrong. That means it might not protect me from being deceived on these basic things either.
10. Now, some people might try to escape this problem by simply denying that God exists. If there is no all-powerful God, then there’s no all-powerful deceiver either. Problem solved, right?
But let’s not get sidetracked by that debate right now. Let’s just grant it for the sake of argument. Let’s suppose that God doesn’t exist. Let’s suppose I got here by pure chance, or fate, or just an endless chain of causes and effects. Say one thing has led to another all the way back for eternity.
Here’s the problem with that. If a powerful, intelligent God didn’t create me, then whatever did create me was less perfect, less intelligent, less capable. And the less capable my creator, the more likely it is that I came out flawed—including flawed in my thinking.
In other words, ditching God doesn’t actually help my certainty. It makes things worse. The less impressive the cause of my existence, the more reason I have to doubt whether my mind works reliably at all.
So, here’s where I’ve landed. I cannot find a single thing I formerly believed with complete certainty that I can’t now find some reason to doubt. I’m not saying this carelessly or rashly. We have found some serious, carefully considered reasons to doubt everything.
From this point forward, if I want to find anything truly certain, I need to treat every belief I have with suspicion, including all the beliefs I was previously very confident about. I should give them exactly the same suspicion I would give to something I knew was completely false.
11. But recognizing all this isn’t enough. I also have to make sure I don’t slip back into my old ways of thinking.
Here’s the problem. Those old familiar beliefs don’t just sit quietly in the corner once I’ve decided to doubt them. They keep coming back. They’ve been with me my whole life. They feel natural and comfortable and reasonable—far more reasonable than doubting everything. And as long as I keep thinking of them as “probably true but slightly doubtful,” they’ll keep sneaking back in and taking over my thinking almost without my noticing.
So, I need a different strategy. Instead of just telling myself these beliefs are doubtful, I’m going to deliberately treat them as if they are completely false. Not because I actually think they’re false, but because that’s the only way to counteract the powerful pull they have on my mind. I’ll fight the old prejudices with a new, deliberate one going in the opposite direction. Only then will my judgment have a fighting chance of finding its way to actual truth.
And before anyone panics, this isn't dangerous. I’m not about to go out and act as if the world doesn’t exist. (That really would be madness.) I’m doing this purely as a thinking exercise. I’m not looking for a guide to action right now. I’m looking for knowledge. And in the search for knowledge, you can’t be too suspicious.
12. So here is my plan. Instead of worrying about whether God might be deceiving me, I’m going to imagine something even more extreme. I’m going to suppose that there exists an evil demon, one massively powerful and completely dedicated to deceiving me in every possible way.
This demon has constructed the entire world I experience as an elaborate illusion. The sky, the air, the earth, colors, shapes, sounds—all of it is just a trap he has set for my mind. I don’t actually have hands, eyes, flesh, or blood. I just think I do because the demon is feeding these false impressions directly into my mind.
I’m going to hold onto this assumption as firmly as I can. And even if this exercise doesn’t lead me to truth, it will at least accomplish something important. It will keep me from being fooled. As long as I maintain this radical suspicion, the demon—whatever his power—cannot get me to agree to something false.
But I’ll be honest. This is hard. There’s a part of me that just wants to go back to my comfortable old beliefs. It’s like being a prisoner who is having a wonderful dream about being free. When he starts to suspect it might just be a dream, he doesn’t want to wake up. He’d rather stay in the comfortable illusion than face the cold reality of his cell.
That’s me right now. I can feel myself wanting to slip back into my old familiar way of seeing the world. Waking up from this comfortable sleep is going to be hard work. And I’m not even sure the effort will pay off. But I have to try.



