It's an obvious question until you try to answer it,

but let's take it seriously.

How do you really know you exist?

In his "Meditations on First Philosophy,"

René Descartes tried to answer that very question,

demolishing all his preconceived notions and opinions

to begin again from the foundations.

All his knowledge had come from his sensory perceptions of the world.

Same as you, right?

You know you're watching this video with your eyes, hearing it with your ears.

Your senses show you the world as it is.

They aren't deceiving you, but sometimes they do.

You might mistake a person far away for someone else,

or you're sure you're about to catch a flyball,

and it hits the ground in front of you.

But come on, right here and now,

you know what's right in front of you is real.

Your eyes, your hands, your body: that's you.

Only crazy people would deny that, and you know you're not crazy.

Anyone who'd doubt that must be dreaming.

Oh no, what if you're dreaming?

Dreams feel real.

You can believe you're swimming, flying

or fighting off monsters with your bare hands,

when your real body is lying in bed.

No, no, no.

When you're awake, you know you're awake.

Ah! But when you aren't, you don't know you aren't,

so you can't prove you aren't dreaming.

Maybe the body you perceive yourself to have isn't really there.

Maybe all of reality, even its abstract concepts,

like time, shape, color and number are false,

all just deceptions concocted

by an evil genius!

No, seriously.

Descartes asks if you can disprove the idea that an evil genius demon

has tricked you into believing reality is real.

Perhaps this diabolical deceiver has dupedyou.

The world, your perceptions of it, your very body.

You can't disprove that they're all just made up,

and how could you exist without them?

You couldn't! So, you don't.

Life is but a dream,

and I bet you aren't row, row, rowing the boat merrilyat all, are you?

No, you're rowing it wearily

like the duped, nonexistent doof you are/aren't.

Do you find that convincing?

Are you persuaded?

If you aren't, good; if you are, even better,

because by being persuaded,

you would prove that you're a persuaded being.

You can't be nothing if you think you're something,

even if you think that something is nothing

because no matter what you think, you're a thinking thing,

or as Descartes put it, "I think, therefore I am."

and so are you, really.