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?

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.

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.

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?

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.
Harald Krull: Steht der links close?
Ich: Ne, der steht rechts.
Harald Krull: Ja, ist ja rechts.