and I'm fascinated by the greatest unsolved mystery in science,

perhaps because it's personal.

It's about who we are,

and I can't help but be curious.

The mystery is this:

What is the relationship between your brain

and your conscious experiences,

such as your experience of the taste of chocolate

or the feeling of velvet?

Now, this mystery is not new.

In 1868, Thomas Huxley wrote,

"How it is that anything so remarkable as a state of consciousness comes about

as the result of irritating nervous tissue

is just as unaccountable

as the appearance of the genie when Aladdin rubbed his lamp."

Now, Huxley knew that brain activity

and conscious experiences are correlated,

but he didn't know why.

To the science of his day, it was a mystery.

In the years since Huxley,

science has learned a lot about brain activity,

but the relationship between brain activity

and conscious experiences is still a mystery.

Why? Why have we made so little progress?

Well, some experts think that we can't solve this problem

because we lack the necessary concepts and intelligence.

We don't expect monkeys to solve problems in quantum mechanics,

and as it happens, we can't expect our species to solve this problem either.

Well, I disagree. I'm more optimistic.

I think we've simply made a false assumption.

Once we fix it, we just might solve this problem.

Today, I'd like tell you what that assumption is,

why it's false, and how to fix it.

Let's begin with a question:

Do we see reality as it is?

I open my eyes

and I have an experience that I describe as a red tomato a meter away.

As a result, I come to believe that in reality,

there's a red tomato a meter away.

I then close my eyes, and my experience changes to a gray field,

but is it still the case that in reality, there's a red tomato a meter away?

I think so, but could I be wrong?

Could I be misinterpreting the nature of my perceptions?

We have misinterpreted our perceptions before.

We used to think the Earth is flat, because it looks that way.

Pythagorus discovered that we were wrong.

Then we thought that the Earth is the unmoving center of the Universe,

again because it looks that way.

Copernicus and Galileo discovered, again, that we were wrong.

Galileo then wondered if we might be misinterpreting our experiences

in other ways.

He wrote: "I think that tastes, odors, colors, and so on

reside in consciousness.

Hence if the living creature were removed, all these qualities would be annihilated."

Now, that's a stunning claim.

Could Galileo be right?

Could we really be misinterpreting our experiences that badly?

What does modern science have to say about this?

Well, neuroscientists tell us that about a third of the brain's cortex

is engaged in vision.

When you simply open your eyes and look about this room,

billions of neurons and trillions of synapses are engaged.

Now, this is a bit surprising,

because to the extent that we think about vision at all,

we think of it as like a camera.

It just takes a picture of objective reality as it is.

Now, there is a part of vision that's like a camera:

the eye has a lens that focuses an image on the back of the eye

where there are 130 million photoreceptors,

so the eye is like a 130-megapixel camera.

But that doesn't explain the billions of neurons

and trillions of synapses that are engaged in vision.

What are these neurons up to?

Well, neuroscientists tell us that they are creating, in real time,

all the shapes, objects, colors, and motions that we see.

It feels like we're just taking a snapshot of this room the way it is,

but in fact, we're constructing everything that we see.

We don't construct the whole world at once.

We construct what we need in the moment.

Now, there are many demonstrations that are quite compelling

that we construct what we see.

I'll just show you two.

In this example, you see some red discs with bits cut out of them,

but if I just rotate the disks a little bit,

suddenly, you see a 3D cube pop out of the screen.

Now, the screen of course is flat,

so the three-dimensional cube that you're experiencing

must be your construction.

In this next example,

you see glowing blue bars with pretty sharp edges

moving across a field of dots.

In fact, no dots move.

All I'm doing from frame to frame is changing the colors of dots

from blue to black or black to blue.

But when I do this quickly,

your visual system creates the glowing blue bars

with the sharp edges and the motion.

There are many more examples, but these are just two

that you construct what you see.

But neuroscientists go further.

They say that we reconstruct reality.

So, when I have an experience that I describe as a red tomato,

that experience is actually an accurate reconstruction

of the properties of a real red tomato

that would exist even if I weren't looking.

Now, why would neuroscientists say that we don't just construct,

we reconstruct?

Well, the standard argument given

is usually an evolutionary one.

Those of our ancestors who saw more accurately

had a competitive advantage compared to those who saw less accurately,

and therefore they were more likely to pass on their genes.

We are the offspring of those who saw more accurately,

and so we can be confident that, in the normal case,

our perceptions are accurate.

You see this in the standard textbooks.

One textbook says, for example,

"Evolutionarily speaking,

vision is useful precisely because it is so accurate."

So the idea is that accurate perceptions are fitter perceptions.

They give you a survival advantage.

Now, is this correct?

Is this the right interpretation of evolutionary theory?

Well, let's first look at a couple of examples in nature.

The Australian jewel beetle

is dimpled, glossy and brown.

The female is flightless.

The male flies, looking, of course, for a hot female.

When he finds one, he alights and mates.

There's another species in the outback,

Homo sapiens.

The male of this species has a massive brain

that he uses to hunt for cold beer.

(Laughter)

And when he finds one, he drains it,

and sometimes throws the bottle into the outback.

Now, as it happens, these bottles are dimpled, glossy,

and just the right shade of brown to tickle the fancy of these beetles.

The males swarm all over the bottles trying to mate.

They lose all interest in the real females.

Classic case of the male leaving the female for the bottle.

(Laughter) (Applause)

The species almost went extinct.

Australia had to change its bottles to save its beetles.

(Laughter)

Now, the males had successfully found females for thousands,

perhaps millions of years.

It looked like they saw reality as it is, but apparently not.

Evolution had given them a hack.

A female is anything dimpled, glossy and brown,

the bigger the better.

(Laughter)

Even when crawling all over the bottle, the male couldn't discover his mistake.

Now, you might say, beetles, sure, they're very simple creatures,

but surely not mammals.

Mammals don't rely on tricks.

Well, I won't dwell on this, but you get the idea. (Laughter)

So this raises an important technical question:

Does natural selection really favor seeing reality as it is?

Fortunately, we don't have to wave our hands and guess;

evolution is a mathematically precise theory.

We can use the equations of evolution to check this out.

We can have various organisms in artificial worlds compete

and see which survive and which thrive,

which sensory systems are more fit.

A key notion in those equations is fitness.

Consider this steak:

What does this steak do for the fitness of an animal?

Well, for a hungry lion looking to eat, it enhances fitness.

For a well-fed lion looking to mate, it doesn't enhance fitness.

And for a rabbit in any state, it doesn't enhance fitness,

so fitness does depend on reality as it is, yes,

but also on the organism, its state and its action.

Fitness is not the same thing as reality as it is,

and it's fitness, and not reality as it is,

that figures centrally in the equations of evolution.

So, in my lab,

we have run hundreds of thousands of evolutionary game simulations

with lots of different randomly chosen worlds

and organisms that compete for resources in those worlds.

Some of the organisms see all of the reality,

others see just part of the reality,

and some see none of the reality,

only fitness.

Who wins?

Well, I hate to break it to you, but perception of reality goes extinct.

In almost every simulation,

organisms that see none of reality

but are just tuned to fitness

drive to extinction all the organisms that perceive reality as it is.

So the bottom line is, evolution does not favor vertical,

or accurate perceptions.

Those perceptions of reality go extinct.

Now, this is a bit stunning.

How can it be that not seeing the world accurately

gives us a survival advantage?

That is a bit counterintuitive.

But remember the jewel beetle.

The jewel beetle survived for thousands, perhaps millions of years,

using simple tricks and hacks.

What the equations of evolution are telling us

is that all organisms, including us, are in the same boat as the jewel beetle.

We do not see reality as it is.

We're shaped with tricks and hacks that keep us alive.

Still,

we need some help with our intuitions.

How can not perceiving reality as it is be useful?

Well, fortunately, we have a very helpful metaphor:

the desktop interface on your computer.

Consider that blue icon for a TED Talk that you're writing.

Now, the icon is blue and rectangular

and in the lower right corner of the desktop.

Does that mean that the text file itself in the computer is blue,

rectangular, and in the lower right-hand corner of the computer?

Of course not.

Anyone who thought that misinterprets the purpose of the interface.

It's not there to show you the reality of the computer.

In fact, it's there to hide that reality.

You don't want to know about the diodes

and resistors and all the megabytes of software.

If you had to deal with that, you could never write your text file

or edit your photo.

So the idea is that evolution has given us an interface

that hides reality and guides adaptive behavior.

Space and time, as you perceive them right now,

are your desktop.

Physical objects are simply icons in that desktop.

There's an obvious objection.

Hoffman, if you think that train coming down the track at 200 MPH

is just an icon of your desktop,

why don't you step in front of it?

And after you're gone, and your theory with you,

we'll know that there's more to that train than just an icon.

Well, I wouldn't step in front of that train

for the same reason

that I wouldn't carelessly drag that icon to the trash can:

not because I take the icon literally

the file is not literally blue or rectangular

but I do take it seriously.

I could lose weeks of work.

Similarly, evolution has shaped us

with perceptual symbols that are designed to keep us alive.

We'd better take them seriously.

If you see a snake, don't pick it up.

If you see a cliff, don't jump off.

They're designed to keep us safe, and we should take them seriously.

That does not mean that we should take them literally.

That's a logical error.

Another objection: There's nothing really new here.

Physicists have told us for a long time that the metal of that train looks solid

but really it's mostly empty space with microscopic particles zipping around.

There's nothing new here.

Well, not exactly.