Eli5: What does ‘I think therefore I am’ mean and why is it a big deal?

Question

Eli5: What does ‘I think therefore I am’ mean and why is it a big deal?

In: Culture

Answer 1

Here’s how it was explained to me, long long ago:

Imagine you’re just a brain in a jar, and aliens have hooked you up to a machine that creates a fake world for you to live in. Everything you know, or have ever known, has been fake. All of the people have just been simulations, all of the History is fiction. How do you know that you’re not fake, too? Well, you’re capable of thinking. And that’s how you know that you’re real and not just another simulation.