✧Hey, Vsauce. Michael here. Where are your fingers? Seriously. It's a pretty easy question. You ✽should be able to answer it. But how do you know? How does anyone know anything?

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✧You might say, well, I know where my fingers are. I'm looking right at them. Or, I can touch ✽them, I can feel them, they're right here and that's good. Your senses are a great way to learn ✧things. In fact, we have way more than the usual five senses we talk about. For instance, your ✽kinesthetic sense, proprioception. This is what the police evaluate during a field sobriety test. It ✧allows you to tell where your fingers and arms and head and legs in your body is all in relation to ✽each other without having to look or touch other things. We have way more than five senses, we ✧have at least twice as many and then some. But they're not perfect.

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✽There are optical illusions, audio illusions, temperature sensation illusions, even tactile illusions. ✧Can you turn your tongue upside down? If so, perfect. Try this. Run your finger along the outer ✽edge of the tip of your upside down tongue. Your tongue will be able to feel your finger, but in the ✧wrong place. Our brains never needed to develop an understanding of upside down tongue touch. ✽So, when you touch the right side of your tongue when it's flipped over to your left side you ✧perceive a sensation on the opposite side, where your tongue usually is but isn't when it's upside ✽down. It's pretty freaky and cool and a little humbling, because it shows the limits of the ✧accuracy of our senses, the only tools we have to get what's out there in here.

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✽The philosophy of knowledge, the study of knowing, is called epistemology. Plato famously said ✧that the things we know are things that are true, that we believe and that we have justification for ✽believing. those justifications might be irrational or they might be rational, they might be based ✧on proof, but don't get too confident because proven is not a synonym for true. Luckily, there are ✽things that we can know without needing proof, without needing to even leave the house, things ✧that we can know as true by reason alone. These are things that we know a priori. An example ✽would be the statement "all bachelors are unmarried." I don't have to go survey every bachelor ✧on earth to know that that is true. All bachelors are unmarried because that's how we define the ✽word bachelor. Of course, you have to know what the words bachelor and unmarried mean in the ✧first place. Oh, you do? Okay. Perfect. That's great. But how do you know?

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✽This time I mean functionally, how do you know? Where is knowledge biologically in the brain? ✧What are memories made out of? We are a long way from being able to answer that question ✽completely but research has shown that memories don't exist in the brain in single locations. ✧Instead, what we call a memory is likely made up of many different complex relationships all ✽over the brain between lots of brain cells, neurons. A major cellular mechanism thought to ✧underlie the formation of memories is long-term potentiation or LTP. When one neuron stimulates ✽another neuron repeatedly that signal can be enhanced overtime LTP, wiring them more strongly ✧together and that connection can last a long time, even an entire lifetime. A collection of ✽different brain cells, neurons that fire together in a particular order over and over again frequently ✧and repeatedly can achieve long-term potentiation, becoming more sensitive to each other and ✽more ready to fire in the exact same way later on in the future. They're a physical thing in your ✧brain, firing together more easily because you strengthen that pattern of firing. You memorized. ✽This branching forest of firing friends looks messy, but look closer. It could be the memory of ✧your first kiss. A living souvenir of the event. If I were to go into your brain and cut out those ✽cells, could I make you forget your first kiss or could I make you forget where your fingers are? ✧Only if I cut out a lot of your brain. Because memories aren't just stored in one relationship, ✽they're stored all over the brain. The events leading up to your first kiss are stored in one ✧network, the way it felt to the way it smelled in different networks, all added up together making ✽what you call the memory of your first kiss.

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✧How many memories can you fit inside your head? What is the storage capacity of the human ✽brain? The best we can do is a rough estimate, but given the number of neurons in the brain ✧involved with memory and the number of different connections a single neuron can make Paul ✽Reber at Northwestern University estimated that we can store the digital equivalent of about 2.5 ✧petabytes of information. That's the equivalent of recording a TV channel continuously for 300 ✽years. That's a lot of information. That is a lot of information about skills you can do and facts ✧and people you've met, things in the real world. The world is real, right? How do you know?

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✽It's a difficult question, but it's not rocket science. Instead, it is asking whether or not rocket ✧scientists even exist in the first place. The theory that the Sun moved around the earth worked ✽great. It predicted that the Sun would rise every morning and it did. It wasn't until later that we ✧realized what we thought was true might not be. So, do we or will we ever know true reality or are ✽we stuck in a world where the best we can do is be approximately true? Discovering more and ✧more useful theories every day but never actually reaching true objective actual reality. Can ✽science or reason ever prove convincingly that your friends and YouTube videos and your ✧fingers actually exist beyond your mind? That you don't just live in the matrix?

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✽No. Your mind is all that you have, even if you use instruments, like a telescope or particle ✧accelerators. The final stop for all of that information is ultimately you. You are alone in your own ✽brain, which technically makes it impossible to prove that anything else exists. It's called the ✧egocentric predicament. Everything you know about the world out there depends on and is ✽created inside your brain. This mattered so much to Charles Sanders Peirce that he drew a line ✧between reality, the way the universe truly is, and what he called the phaneron, the world as ✽filtered through our senses and bodies, the only information we can get. If you want to speak ✧with certainty you live in, that is you react to and remember and experience your phaneron, not ✽reality. The belief that only you exist and everything else, food, the