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?