How do you know you're not dreaming?

You’re a butterfly, fluttering around pursuing a butterfly’s whims. Then you wake up. But how do you know you’re not dreaming now? The answer might seem obvious, but it’s actually very difficult to explain how, definitively, you know you’re awake. So difficult, in fact, that it has puzzled philosophers since ancient times. In the butterfly scenario, the ancient Chinese philosopher Zhuangzi surfaced a mystifying possibility: if we can dream of being an entirely different creature, who's to say we're not actually a different creature dreaming of being human? Bizarre things happen in dreams: you fly, or conjure an all-you-can-eat dessert buffet out of thin air, or get chased by witches through the halls of your elementary school, which suddenly looks a lot like Paris. But the strange things that happen in dreams don’t seem strange at the time. So how do you know you’re not in a dream right now that will seem very strange after you wake up? Well, it is possible to notice the strangeness of a dream while you’re dreaming. Lucid dreamers know they’re dreaming. By definition, if you were having a lucid dream, you would know it. But all that proves is that you’re not having a lucid dream— it doesn’t prove you’re awake. There has to be a surefire test— something that never— or only— happens when you’re awake, something that never— or only— happens in a dream. Wake up. No, that isn’t it— you can wake up in a dream. Pinch yourself. If it hurts, aren’t you really awake? Try to read or write something. Run around the room. Does your pace seem normal or suspiciously slow? Suspiciously fast? Can’t tell? Try to remember the last time you ran. Actually, that brings us to an even better test from the 17th century French philosopher René Descartes. He pointed out that in our memories, dreams are disconnected— the events of a dream don’t fit in to the chain of events in our waking lives. This seems rock solid, doesn't it? You couldn’t possibly have swum with dolphins in a nameless pink sea between Christmas and New Year’s Eve because you didn’t leave Kansas and you have the receipts to prove it. Well, one of Descartes’ contemporaries, the English philosopher Thomas Hobbes, had something to say about that: what if Descartes was performing his test in a dream? . What if we ask an expert? A neuroscientist can measure the activity in different parts of your brain and tell you whether you’re awake or sleeping. But that just brings us back to the idea that any test you might use to prove you’re awake could take place in a dream. So far, no one has found a convincing response to this. But let’s be real: there’s a whole lot more detail in our waking experience than in dreams. We go to sleep and wake up again day after day for many years, and each new day is full of countless people, places, things, experiences. Even our memories, which capture just a fraction of this experience, contain an almost incomprehensibly vast amount of detail: we can recall a line from a favorite book decades later, remember the musty smell of its pages and the taste of the lemonade we drank while reading it, remember a dream we had about it and tell someone all this. Isn't it ridiculous to suggest a dream could ever simulate this richness? Well, as the Persian philosopher al-Ghazali pointed out, in the same way we think we are now awake having woken from dreams, it is possible that we might wake from our current state into another state of even greater wakefulness. Which would mean we’re really in a kind of dream-state when we think we’re awake. What philosophers really want to know is what justifies our belief that we’re awake. We all want to believe things because we have reasons for them, not just because they seem right. Sometimes, the biggest challenge is to show why we should believe something that seems completely obvious to us all.