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A film for your philosophical consideration: The Matrix. You've got to remember—the humans floating around in the vats of K-Y Jelly, tubes and wires keeping them alive, stimulating their brains to make them believe that they were experiencing the real world, the world we all think we know. Well (almost 20-year-old spoiler alert here), some of them come out and find that the real world was a desolate wasteland, and the lives everyone thought they were living were just fabrications fed into their brains.

A select few were rescued from the illusion, but some of them were so unhappy in the real world that they chose to return to the illusion. But Neo and the others who chose to stay and fight were the philosophical heroes of the movie, choosing truth at the cost of comfort and happiness.

After watching The Matrix, you might have found yourself wondering, "Could this be true? Could we possibly be stuck in a dreamworld of someone else's making, with no way to tell that our reality isn't real at all?" If so, you are not the first person to have wondered about these things. In fact, the original Neo, the guy who really went into battle against the matrix of illusion in order to defend the truth, he was a 17th-century mathematician named René.

[Crash Course Philosophy intro]

Last time, we talked about Plato and his belief that the ordinary reality of the material world is only a shadowy approximation of ultimate reality. Socrates, meanwhile, who was widely believed to be the wisest man in Athens, fretted about how little he knew.

Philosophers spend a lot of time obsessing about knowledge, wishing they knew more, and worrying that they're wrong about what they think they know. They even—if you remember from the first episode—have a fancy name for the study of knowledge: epistemology. The philosopher who gets the gold star for taking this how-do-I-know-what-I-know paranoia to astonishing levels is the early modern philosopher, scientist, and mathematician René Descartes.

When you watch The Matrix, you should congratulate the Wachowskis for giving us such a great sci-fi adventure story. But you should also remember that the archetype of the story actually has its roots all the way back in the writings of Descartes in the early 1600s.

For a story like The Matrix to get off the ground, the audience has to be willing to entertain some level of skepticism. And a skeptic is someone who questions whether it's possible to know anything with certainty. And Descartes was the mack daddy of all skeptics. He was so skeptical, we named a form of skepticism after him: Cartesian skepticism.

Why was Descartes so skeptical? Well, he realized that many of the beliefs he used to hold were actually false. We all go through this; it's part of what we call "growing up"—learning those horrible truths about Santa and the Tooth Fairy; that you can't actually buy everything you want and need for just $100; that your parents don't really have all the answers.

But realizing that he used to believe things that were false really got Descartes to thinking. Because when he believed those things he didn't realize they were false, so what if some of the things he still believed were also false and he just hadn't realized it yet? How could he know that his beliefs were true?

Well, after a bit of a freak-out, Descartes realized that the only way to make sure he wasn't holding any false beliefs was to disbelieve everything, at least temporarily. He offered this as an analogy: Imagine you have a basket of apples and you're concerned that some of the apples might be rotten. Since the rot can spread and ruin the fresh apples, the only way to make sure there's no rot in the basket is to dump out all the fruit, inspect each apple in turn, and return only the fresh apples to the basket.

Knowing that just like rotten fruit a rotten idea can spread and infect all the ideas around it, Descartes upended the apple basket of his beliefs and decided to start from scratch. If he examined each possible belief carefully and only accepted those about which there could be no doubt, then he'd know he was only believing true things. So Descartes began the arduous task of examining his beliefs, one by one.

He started with empirical beliefs—things we come to know directly, through the use of our senses. And many of us think that our senses are the most reliable source of information. If I can see it, hear it, touch it, taste it, smell it, I must know it, right? Not so much.

Descartes pointed out that our senses fail us all the time—you rush to catch up to a friend and realize as she turns around that your eyes played some tricks on you and you've just tapped on the shoulder of a perfect stranger; food tastes wrong when you're sick; drink to much and you feel like the room is spinning; water that's room temperature feels hot when you come inside after playing in the snow. This list goes on. You can probably think of countless times where your senses gave you faulty information. And once you realize that, how can you ever trust your senses again?

And for Descartes, disbeliever of everything, it got worse. Have you ever had a dream so vivid you thought you were awake? You've probably had that dream when you were dreaming you were dreaming, or you dreamed that you woke up from a dream but in fact were still in the dream. Not everyone has had these experiences, but many of us have, and given that we don't always know that we're dreaming while it's happening, how do we know we're not dreaming right now? Maybe you just think you're watchin Crash Course, but in fact you're cozied up in bed, dreaming about me. Which, hey, who could blame you? But really when you think about it, can you be sure that it's not the case?

Now, you might be thinking, "Okay, sure, I probably deceive myself from time to time without knowing I'm doing it, but dreams end, and when I wake up I realize that what I thought I was experiencing was all in my head. And the same is true for when my senses let me down. Those are just temporary instances, isolated to a particular situation. As soon as the situation changes, I can realize that my experience was false."

This quality, the ability to check in with yourself and figure out that you're experiencing a deception, describes what Descartes called "local doubts". Those are doubts about a particular sense experience or some other occurrence at a particular point in time. Step out of that point and you can check to determine if you've been deceived.

But what if everything is a deception? What if everyone is experiencing the same false reality from birth until death? What if nothing is as it seems, just like in The Matrix? This type of doubt, the kind you can't step out of and thus can't check, is called "global doubt", and it's the subject of this weeks Flash Philosophy. Let's go to the Thought Bubble.

Philosopher Bertrand Russell illustrated the concept of global doubt with this troubling thought: What if the universe was created just five minutes ago? In this scenario, known as the Five Minute Hypothesis, the creator of the universe could have designed many elements of the world to make them appear pre-worn so as to seem old—from dinosaur bones fashioned by the creator and planted for us to find, to that scar on your knee put there by the creator along with the preloaded false memory of how you got it. It seems crazy, but there's no way to prove that it isn't the case. The question for Russell was, does it matter?

Descartes thought it did, but as a good Catholic he couldn't fathom a world in which God would plant globally false beliefs in all of our minds. So instead he posited the existence of an evil genius whose purpose in life was to deceive us and who was clever enough to do it. Descartes didn't actually think such a being was likely to exist, but he realized there was no way to rule out his existence. And as long as the evil genius was possible, he worried that we were all stuck, stuck in a radical skepticism in which we really can't trust any of our beliefs.

Everything we believe—every sense experience, every thought—they could all have been put in our minds by the evil genius who created an illusory world so seamless we'd have no way of detecting the illusion. Just like the machines created for the characters in The Matrix. Descartes was at the point of despair. But then he realized something. He had cause to doubt everything, everything except the fact that he was doubting.

He knew he was doubting; he could be sure of that. And if he was doubting, then he must exist, at least as a thinking thing. After all, a doubt is a thought, and if there is a thought there must be a thinker having those thoughts. So Descartes decided that he couldn't know that he had a body—what he believed to be his body could have been part of the evil genius's deception. But he must have a mind, or he couldn't be having these thoughts. This was Descartes's aha moment. In his book Meditations on First Philosophy, he declared, "Cogito ergo sum. I think, therefore I am."

It's one of the most famous realizations in philosophy: I cannot doubt my own existence. I can doubt everything else, but I can't doubt that I am, bare minimum, a mind having thoughts. This was Descartes's foundational belief, the first belief he put back in his apple basket. And from there, he figured he could build back up to more certain beliefs.

Once he was certain that he was a thinking thing, he began examining his thoughts. And one of his most clear thoughts, what he called a "clear and distinct idea", was that God exists. He gave an argument for this, which we're going to examine in a later episode. But for now, take my word for it, it's got some problems.

And from there, he considered his beliefs about the physical world and concluded that it, too, actually exists. Ultimately, he determined that God wouldn't allow him to have clear and distinct ideas that were false without some way to detect his own error, so he concluded the evil genius is not actually fabricating lies that consume our every waking moment. Descartes managed to reason from "cogito" all the way back up to having basically all the beliefs he started with back in his apple basket, which is the story of how René Descartes, with the power of skepticism, defeated the threat of the evil genius. Much like how Neo ultimately short-circuited the Matrix, though considerably less impressive to watch, I imagine.

He found certainty through his discovery of the one belief that he simply couldn't doubt—his own existence as a thinking thing. But there is a lot of debate among philosophers as to whether Descartes actually manages to justifiably believe anything other than that he exists as a thinking thing. And we'll talk more about that next time.

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Crash Course Philosophy is produced in association with PBS Digital Studios. You can head over to their channel to check out amazing shows like Deep Look, The Good Stuff, and PBS Space Time.

This episode of Crash Course was filmed in the Dr. Cheryl C. Kinney Crash Course Studio with the help of all of these amazing people, and our graphics team is Thought Café.