is a surgical procedure in which half of a person's brain

is removed.

It's usually only ever done on very, very young patients

because their brains are still plastic enough

that the remaining half will take on the functions

of the half that was removed.

And it's usually done because a young child or a baby

is having seizures, and removing the part where the seizures occur

is the only solution.

But here's my question.

If you can live with half a brain,

what if I were to take two empty skulls

and take one half of your brain

and plop it into one body and the other half

and put it in another body,

which person would be you?

I mean, you are you. You are conscious.

You are aware of what is happening to you

from the perspective of yourself. Think of it this way.

If you just stare at something and kind of feel

what it feels like to be you,

it feels a little bit like you're a thing

inside a body looking out through the eyeballs.

And nobody else on Earth

will ever see the world from that position.

This awareness of your own experiences,

the awareness that you are having them,

the awareness that you are having your own thoughts

makes up what we call consciousness.

But if I were to take your brain and split it into two

and put it into two different people,

would both of them be new people who were conscious?

Well, one of the best places to start

when defining consciousness and understanding it

is to begin with things that we agree are not conscious.

For instance, Cleverbot.

Cleverbot.com is an amazing website

where a computer program will respond to your questions really cleverly

but only because it is programmed to do so.

We wouldn't consider it conscious

because it doesn't have a sense of itself.

It doesn't feel anything. It doesn't have its own inner life.

It's just a program that responds automatically to my inputs.

Now I know that I am not like Cleverbot.

I know that I feel things and that I have a sense of myself.

I have intentions.

But how do I know that you do?

For that matter, how do I know that everybody else that I meet is like me?

How do I know that they're not just little smart versions of Cleverbot

who know exactly what to automatically say?

Now what I'm asking is incredibly philosophical,

but it is a very famous and important question.

I'm basically asking if it's possible for something to exist

as a philosophical zombie.

That's right, a thing that reacts and responds and acts just like a normal human

but yet doesn't actually feel anything.

It doesn't know that it's having its own thoughts.

It just automatically responds like a robot in the appropriate way.

Now what's amazing and heavy about this question

is that science doesn't have an answer,

and it's not even clear that science will ever have an answer,

let alone an approach to finding that answer.

About all we have is the psychology of disorders of consciousness.

Let's begin with anosognosia.

A common example of anosognosia in psychology classes

is a patient who has, say, lost the ability to move their left hand.

When asked to raise their right hand,

they'll say, "Yeah, no problem, here you go."

But then when asked to raise their left hand,

they'll say, "Oh, yeah, sure, no problem,"

but not move it. And when asked why they didn't move their left hand,

instead of reporting that they can't,

they'll confabulate some excuse.

For instance, "Oh, I didn't feel like it."

Anton-Babinski Syndrome is even more dramatic.

Patients with this syndrome are cortically blind.

They cannot see anything.

But they deny being blind.

If you ask them a question, for instance, "How many fingers am I holding up?"

they will make a guess, but if they're wrong,

they'll explain their inaccuracy with an excuse.

For instance, "oh, well, I don't have my glasses."

People who exhibit anosognosia

tend to be the victims of stroke,

and there's some disconnect between what they're really experiencing

and their conscious awareness of it.

They don't know that they can't see

because the part of their brain that monitors visual input

isn't telling the brain anything.

It's not even telling the brain that there is no visual input

which means that the parts of their brain

responsible for answering questions or creating speech

has to completely create a confabulated response.

Despite the fact that we've been able to study

patients with anosognosia, we still have no idea

how to solve our original problem.

In fact, all we've managed to come up with

are more impossible questions about identity,

questions that are so befuddling, the best you can do with them

is to answer them yourself according to what you believe.

Here's another one. It's called the Swampman.

Imagine that I'm walking around in a swamp

and then all of a sudden, I get struck by a bolt if lightening

and my entire body is burned to a crisp, dissolved into smithereens.

But the very same moment, a second bolt of lightening strikes nearby,

and it causes a bunch of atoms and molecules

to all arrange themselves into the exact same configuration

that my body used to have, making a second Michael.

Is that me? Would that be me?

Here's an even better one. Imagine that a surgeon came in,

and he started removing cells from me and from you,

replacing them exactly one at a time,

replacing my cells into your body and your cells into my body.

At what point would I officially have become you?

No one on Earth has the definitive answers to these questions,

but you know what we do have? A lean back.

That's right. I've made a playlist of some of my favorite clips from all over YouTube

of psychology experiments and illusions

and all sorts of fun stuff that deal with consciousness.

All you have to do is click the link at the top of this video's description

and then lean back and let the automatic playlist do the work for you.

I'll see you over there, and as always, thanks for watching.

Hey Kevin, any new YouTube messages?

Actually yes. I just got this message over at Vsauce 2

from a user who doesn't create videos

but he organizes them in his own playlists.

Yeah, yeah, that's excellent because there's so many videos on YouTube,

we need people to help organize them, especially by type.

Imagine a channel of playlists ranging from cool songs to listen to when it rains

or best explosions ever.

Exactly. If you make a playlist that you think is really cool

and full of stuff that we should know about, send it to Michael@Vsauce.com

and we'll feature the ones we really like

because you have made YouTube a better more organized place.

And after all, Vsauce is kind of like a carpool lane.

We're all going to get to the cool stuff faster

because we're traveling together.

Where did you get that?