How do you know you exist?

How do you know you're real?

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 duped you.

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 merrily at 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.

(Airplane engine)