Cogito ergo sum.

I think, therefore I am.

Descartes said that. Rene Descartes laid the foundation of modern philosophy. Because he was badass like that.

Many people quote that line. They put that on T-shirts. But I do sometimes wonder about how many people actually think (har!) about what that sentence really means.

Here's the thinking (har!) behind that. Basically, Descartes said, you cannot trust anything. When it comes to deriving knowledge about something, you cannot trust anything. You cannot trust your senses. Senses can be deceived. Just watch a fucking magic show. So how do you know anything is real? How do you know that *you* are real? You perceive you through your senses and through the senses of others, all of which could be an elaborate cosmic deception. You cannot trust science because science uses instruments to establish reality. And those instruments are simply extensions of your already untrustworthy senses.

So how do you know that you exist?

By doubting, that is how. This is pure rationalism at work. When you doubt your existence, you establish your existence. Your doubt exists, therefore you exist. If you didn't really exist, you couldn't doubt your existence. Q. E. fucking D.

Still not with me? Go back to that magic show analogy and approach this from another angle. The only way you know that the magic is magic is by doubting it. If you didn't doubt the magic, it wouldn't be magic to you any more; it would be reality. And you would be wrong. (Which, by the way, is how all religions work). Now apply that line of thinking to the question of your existence and you immediately grok what the Big D was getting at.