If you've ever so much as dipped your toe in the vast ocean that is philosophy,

then you will have heard the phrase "I think therefore I am,"

first written by 17th century French philosopher and Captain Hook lookalike René Descartes.

In his "Meditations on First Philosophy," Descartes tried to find absolutely certain knowledge,

and what he realised was that a lot of what he thought he knew was actually open to doubt.

For instance, I think I'm making a video right now but I could be asleep and dreaming,

or hallucinating, or living in a virtual reality.

Descartes was especially concerned with the possibility that he might be dreaming his entire life.

He famously wrote,

"There are no definitive signs by which to distinguish being awake from being asleep."

Now, you're probably thinking,

"That's a stupid thing to be worried about. I know I'm not dreaming

because I'm not flying or Batman or having dinner with Olivia Wilde."

But the point is that when you're dreaming, most of the time you think it's real

and you think this is real, too, so how do you know you aren't dreaming right now?

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Descartes actually took this one step further.

See, in a dream you can still know things like 2+2=4 and all triangles have three sides.

These are just truisms, tautologies.

But Descartes chose to imagine an all-powerful, evil demon

whose sole purpose was to trick him about everything he thought he knew.

Everything that you think you know, the evil demon could be deceiving you with his fantastic powers.

Now, that sounds like a bit of a crazy idea and, yeah, a lot of philosophy is a bit pie-in-the-sky.

We normally assume that we know things because if we had that kind of scepticism all the time

then we just wouldn't get anything done.

But Descartes wasn't happy with things that seemed true or were probably true.

He wanted to be absolutely certain, so he tried to doubt everything he could.

If there was even the slightest chance that something might be false,

he disbelieved it in order to find something indubitable.

He disbelieved in the external world, in his own body, in other people, everything,

to try and find something certainly true,

and what he found was that he couldn't doubt his own existence.

Just the act of doubting it confirmed that there was someone there doing the doubting.

Hence, "I think therefore I am."

Now, Descartes went on to build a whole lot of other stuff on top of that,

but we're gonna stick with that for now because it might not be as indubitable as Descartes wanted it to be.

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Descartes doesn't do a great job of spelling it out,

but what he's driving at is that he can't doubt that he is thinking since doubting itself is a kind of thinking,

and, since thoughts require a thinker, that's how he knows he must exist.

But 19th century German philosopher and mustachioed badass Friedrich Nietzsche --

Neetchuh. Neetchuh. Not neetchee. Neetchuh. --

thought that "thoughts require a thinker" isn't indubitable.

Thoughts might exist as some kind of free-floating chain of unconnected ideas,

so the most that Descartes could say was that thinking was going on.

So there's that to think about, but there is also another question, too, namely "Does indubitable mean true?"

It seems like a lot of things could be indubitable.

I could find it indubitable that I'm Napoleon but that doesn't make it true.

But it's important to remember what Descartes means when he uses the word "doubt" here.

When he says he's going to doubt anything that isn't indubitable in order to find certain knowledge,

what he means is he's going to disbelieve anything that might be wrong.

If I'm totally convinced that I'm Napoleon, then I might at least admit that I could be wrong about that.

Heck, even Napoleon himself might entertain the thought that he could be wrong about his identity.

But if I think that I'm thinking, I can't be wrong about that, since, if I was,

then I wouldn't be considering the problem.

The issue is whether or not I can say that it's definitely me doing the thinking,

i.e. do thoughts require a thinker?

Nietzsche and Bertrand Russell thought that there was at least a chance Descartes could be wrong about that.

Bertrand Russell was the one who suggested that thoughts might exist as a chain of unconnected ideas,

but that's kind of a weird idea -- like, what would thoughts even be if there was no one there having them?

What do you think? Leave a comment with your suggestions and questions

and we'll answer them at the end of the next video! Favourite, share, tell your friends about Philosophy Tube,

and, if you leave a like, then you definitely exist.