how we actually know anything, Gettier's problem, rationalism vs. empiricism, and Descartes

Here's a question that sounds childish until you actually try to answer it. How do you know the chair you're sitting on is real, and not some incredibly convincing illusion your brain is generating? You'll probably say something like "I can see it, I can touch it, obviously it's real." But your senses have fooled you before. Dreams feel completely real while you're in them. Optical illusions trick people who know exactly how the trick works. So what makes you so sure this moment is different?

This is the territory of epistemology, the branch of philosophy concerned with knowledge itself. Not what we know, but how we know it, and whether "knowing" something even means what we assume it means.

The Problem With "I Just Know"

Most people walk around with an unspoken assumption that knowledge is simple. You either know something or you don't. But philosophers have spent centuries picking apart what "knowing" actually requires, and the standard answer looks something like this: knowledge is justified true belief. To know something, you need to believe it, it needs to actually be true, and you need good reason to believe it.

That sounds reasonable until you hit a famous problem, first raised seriously by the philosopher Edmund Gettier in 1963. Imagine someone has a broken clock that happens to show the correct time twice a day. You glance at it at exactly the right moment and form the belief that it's 3pm. It really is 3pm. Your belief is true, and you had reason to believe it, since you looked at a clock. But something feels off about calling this "knowledge." You got the right answer for the wrong reason, by pure luck. Gettier used cases like this to show that true belief plus justification isn't quite enough. Something else is missing, and philosophers still argue about what that something is.

Where Your Certainty Actually Comes From

There are basically two camps on where knowledge comes from in the first place.

Rationalists, like Descartes, argued that some knowledge comes purely from reason, independent of the senses. Math is the classic example. You don't need to touch or see anything to know that 7 plus 5 equals 12. You can work it out through logic alone, sitting in a dark room with your eyes closed.

Empiricists, like John Locke and David Hume, pushed back hard on this. They argued nearly all knowledge ultimately traces back to sensory experience. Even abstract ideas, they said, are built from combinations of things we've seen, heard, or felt at some point. You can imagine a golden mountain without ever seeing one, sure, but only because you've seen gold and you've seen mountains separately, and your mind combined them.

This split matters more than it sounds. If empiricists are right, then everything you claim to know is only as reliable as your senses, and your senses are demonstrably unreliable. If rationalists are right, some truths are available to any mind capable of reasoning, independent of a shaky nervous system. Neither camp has fully won the argument, and most modern philosophers land somewhere in between.

Descartes Doubts Everything

The most famous stress test of all this came from Descartes himself. He set out to doubt every single belief he held, to see what, if anything, survived. He doubted his senses, since they'd deceived him before. He doubted his memories, since dreams feel just as convincing as waking life while they're happening. He even entertained the idea of a powerful deceiver, some evil demon capable of feeding him false experiences at every turn.

What survived this brutal process was a single thought: the fact that he was doubting at all proved something was doing the doubting. Even if every one of his beliefs was false, the act of thinking required a thinker. That's where his famous line comes from, "I think, therefore I am." It's often treated as a throwaway quote, but in context it was the last stone left standing after he tried to demolish everything else.

The catch is that this only gets you one certain fact: that you exist as a thinking thing. It doesn't get you the chair, the room, or anyone else in your life. Descartes spent the rest of his work trying to rebuild outward from that single point, with mixed success depending on which philosopher you ask.

Why This Isn't Just Navel Gazing

It's tempting to file all this under "fun to think about, doesn't change anything." But it quietly shapes real decisions.

Think about how often people say "I just know" as if that settles an argument, whether it's about a relationship, a business decision, or a conspiracy theory they've fallen into. Epistemology forces a follow up question: know how? Based on what? Could you be the guy checking the broken clock at exactly 3pm?

It also shows up in how seriously you should take expert consensus versus personal intuition. Scientific knowledge is built on a process specifically designed to catch the kind of lucky guesses Gettier described, through repeated testing, peer review, and demands for evidence that doesn't rely on a single observer's gut feeling. That doesn't make experts infallible, but it does mean their claims usually rest on sturdier ground than a hunch.

And on a more personal level, it's worth occasionally asking yourself which of your strong beliefs are actually justified, and which ones you just absorbed somewhere along the way and never questioned. Not everything needs to survive Descartes level scrutiny. But a little bit of that doubt, applied honestly, tends to make people harder to fool.

Living With Uncertainty

None of this means you should spiral into doubting whether the chair beneath you is real every time you sit down. Descartes himself didn't live that way. He used radical doubt as a tool to find solid ground, not as a permanent lifestyle. Most working philosophers today accept a version of knowledge that's good enough for practical life while staying honest about its limits.

Maybe that's the real takeaway. Certainty in the absolute sense might be out of reach. But there's a difference between having perfect certainty and having good enough reasons, tested against reality, held with the humility to change your mind if better evidence shows up. That's not a weakness in how we know things. It might be the most honest description of what knowing has ever meant.