Here is a hand. How do I know? Look closely, asshole, it's clearly a hand.

Look, if you really insist on doubting that here is a hand, or anything else, there's nothing really I can say to convince you otherwise. What the tits would the world even look like if this weren't a hand? What sort of system is your doubt endorsing? After all, you can't just say "It's not true that here is a hand." You have to be endorsing some other picture of the world. [...]

So it turns out when I say things like "Here is a hand" I'm not really making a claim about the world, I'm laying down some rules for discussion. If you doubt there's a hand here, then fuck you and that's all there is to it. We can't really talk about anything now, because we can't even agree on something as simple as a goddamn hand. When we all agree here is a hand, then we can go about discussing our world in meaningful ways. Skepticism just undermines a foundation and replaces it with nothing; it[']s paralyzing. The grounds for such radical skepticism don't exist; it presupposes and relies on the very certainty it tries to undermine.

This is more practical than you realize. There are people who actually believe that the world is only 6,000 years old. What the fuck, right? But if you've ever talked with one of them, you know that they're fucking impossible to have what you consider a 'reasonable' discussion with. It's not like they don't have answers for everything, it[']s just that those answers don't make any fucking sense to you. It[']s the sort of gibberish that makes you want to scream. The problem is that you don't even play the game by the same goddamn rules. You're both certain of your positions, because those positions are logically derived from the worldview each of you endorses as your starting point, and you both look at each other's foundations and say, "Seriously, what the fuck are you talking about?" You don't even know how you would go about convincing them that you're right and they're wrong; you don't even agree on a method by which to do that.

If you flew to some part of the world where they'd never heard of an airplane or even a bird, how the fuck could you convince them you flew? They don't even know what that means. They would have all sorts of questions, and would consider your answers nonsensical or magical. When a non-believer is told that God exists, he reacts in the same way; also, a believer when he is told there is no God.

So everything we believe about the world is built on some sort of foundation. Sure, that foundation can change, but there is always something there at the base, and it is that base that enables us to talk about the world. Not everyone has the same base you do, and that has to be okay. Just know that some of your beliefs are just as unsupported as everyone else's. It's just the way it is, bro.

— Philosophy Bro summarizing Wittgenstein's "On Certainty". (I'm not sure the summary is very true to the original but it's interesting nonetheless.)