Who Are We? The Story of Science: Power, Proof and Passion

There are some great questions

that have intrigued and haunted us since the dawn of humanity.

What is out there?

How did we get here?

What is the world made of?

The story of our search to answer those questions

is the story of science.

Of all human endeavours,

science has had the greatest impact on our lives,

on how we see the world, on how we see ourselves.

Its ideas, its achievements, its results are all around us.

So how did we arrive at the modern world?

The answer is more surprising and more human than you might think.

It is a tale of power,

proof,

and passion.

This time, one of the more intimate questions we've ever asked.

What makes us human?

The question, what is human nature,

what is it that shapes our thoughts, feelings and desires,

is one that philosophers, writers and religious leaders have all struggled with.

I am particularly interested in how science has wrestled with this particular question,

and that's not just because it gets to the heart of who we are,

but also because it gets to the heart of what science itself is.

I want to begin with one of the great civilisations of the ancient world -

Egypt.

The ancient Egyptians were amongst the first people we know about

to really wrestle with the question, what makes us human?

We humans are acutely aware of ourselves,

of the sense of being alive, of living within our own skin.

But where does this "me" reside?

Where is the control centre?

Where is the essence of what I truly am?

Egyptian beliefs about what made us human

are revealed in their attitudes to the afterlife.

Certain organs, like the stomach, lungs or liver,

were seen as so critical they were frequently removed, embalmed,

and put back inside the body for burial.

The Egyptians believed that the heart was the key to the afterlife,

that when you died it would testify for your good or your bad deeds.

On this papyrus you can see a heart being weighed up against a feather.

If it was heavier than the feather then this demon over here

would come and eat it, and that was all over for you.

In fact, the idea of being light-hearted or heavy-hearted comes from the Egyptians.

And in a way you can understand why they thought that the emotions resided in the heart.

Certainly when I have been broken-hearted I've felt it in my gut, and in my chest.

So the Egyptians treated the heart with great reverence.

But what about that other organ we now regard as more central to our humanity?

Here at Manchester University,

a team of Egyptologists are studying a 2,500-year-old mummy.

An endoscope is going to be pushed up its nose

to show me how the Egyptians treated the brain.

Carefully.

As we enter the nose

through the nasal septum...

How extraordinary.

-It's like going into some sort of hidden cave.

-It is, isn't it?

It's a secret world, really.

We would normally be stopped from going through there because of the bone

that would separate the brain from the nasal cavity.

-Which should be there.

-Yes, it should be there, of course.

-Right. And so now you're actually entering the skull?

-Yes.

Ooh!

That's a sort of, a suture in the top of the head, isn't it?

There seems to be something missing.

Yes, there's a brain missing.

How extraordinary.

Do they not see the brain as important?

They recognised that the brain controlled some of the bodily actions,

but they certainly didn't think that the individual personality

was located in the brain.

So they removed it and discarded it.

-So they just took it and chucked it out.

-Yes.

-It shows a certain contempt for what we regard as one of our more important organs now.

-Absolutely.

The Egyptian concept of what makes us who we are was a mystical union

between the physical body and an everlasting spirit.

One of the recurring ideas to emerge out of early civilisations like the Egyptians

was the belief that we are more than simply flesh and blood.

There is something else, something which is special and makes us human.

This conviction is one of the most powerful and enduring in human history.

This belief shapes thinking for millennia.

But as Europe emerged from the Middle Ages,

people started to approach the question differently.

The physical and intellectual frontiers of Europe were changing,

and that would encourage a very different view of who we are.

That new view can be glimpsed here, the grandest royal palace in France.

Amongst this great splendour, there's an intriguing technology...

..that to me reflects a great change in how we saw ourselves,

captured in one magnificent room.

And this is it. It's the great hall of mirrors in Versailles.

It is absolutely fantastic,

and the whole room utterly dominated by this wall of mirrors

which extends down almost 100 metres.

I've never seen mirrors on this scale.

This really is cutting-edge technology.

Now this is not absolutely perfect, the surface not completely smooth,

you can see little bubbles here in the glass.

It's not perfect, it's not like a sort of modern mirror.

But the size and the scale is unlike anything

which was really done before,

and compared to the sort of curvy-wurvy things

that most people would know of from centuries earlier,

this was something different.

Because there was nothing, nothing, nothing like this had been developed before.

It allowed people to just stand there

and look at themselves and think, you know, "Who am I?" "This is me."

These mirrors represent the culmination of an idea

that had been emerging in Europe since the Renaissance.

The notion that we are all individuals.

Not members of a class, or a guild,

but defined by our own desires, ambitions, and destinies.

Along with this growing awareness of self came different questions.

What makes ME who I am?

Why do I have these hopes, these fears, these talents, these expectations?

And most importantly of all, what is this "I" anyway?

Throughout history, the technology of the age has stimulated new ways of looking at the world.

I can see a thing which looks a little bit...

I don't know what it is, it looks like some sort of sea creature, possibly a prawn.

New inventions have created metaphors to help us think about what makes us human.

This makes me smile.

In 17th-century France, the philosopher Rene Descartes

was wrestling with the question of human nature.

For inspiration, he drew on a technological wonder of the age -

water-powered mechanical statues.

The story goes that Descartes is wandering through the royal gardens

and he sees a fountain, and in the middle of the fountain there is an enormous statue of Neptune,

which is spouting water, a bit like this.

And this particular Neptune, when you come close, sort of starts to jab at you with the trident.

And Descartes is rather taken by this, and he starts to think,

and he thinks perhaps animals are just a form of automata,

that perhaps a prawn really has some sort of gears in it

with lots of sort of intersecting bits and pieces.

And then he starts wondering, perhaps that's what our bodies are,

they're just sophisticated machines.

For the time this was a very daring idea,

to suggest we are like machines,

but it begged the question, what special quality actually makes us human?

Descartes was a man desperate for certainty,

but this was no time to find it.

17th-century Europe was riven by religious and political conflict.

Old certainties of Church and State were crumbling.

What, thought Descartes, could he trust?

What could he really know?

Descartes is wracked by doubts,

and he wants to find out something he can believe in.

Imagine, says Descartes, a tower,

and the tower is in fact round, but you perceive it as square.

Or, for example, this thing here - from a distance it looks square

but actually when you hold it up it is clearly round.

Your vision has been deceived.

And then Descartes wondered if all his senses were deceiving him.

He could feel the warmth of his fire,

see its light, hear its sound,

but he'd experienced the same sensations in a dream.

So perhaps the whole world he was living in was nothing but an illusion.

Descartes is now beginning to really question everything -

the moon, the sky, the stars.

Perhaps they're all figments of his imagination. But what about maths?

Two plus three - it always equals five, doesn't it?

But maybe there's a demon who's taken possession of his brain.

Descartes is really beginning to doubt everything,

down to the very question of whether he himself existed at all.

And then, finally, he got there.

He realised that the act of doubting implied a doubter.

There was one thing he could be absolutely certain of -

the existence of his own thinking, doubting mind.

He summed it up in a neat philosophical phrase - "I think, therefore I am".

It may be a familiar phrase, but it contains a profound idea -

the claim that the essence of our humanity lies in our thoughts,

our ability to reason.

And reason was to form the basis of a new, experimental science.

Across the Channel, a much more bloody approach to the question of "Who are we?"

was to emerge from a great political clash -

the English Civil War.

Oxford was a key Royalist stronghold.

For some caught up in the action, turmoil spelt opportunity.

Here in Oxford, a young man called Thomas Willis was part way through

his medical training, which in those days lasted an incredible 14 years.

The Civil War interrupted his studies,

which in many ways was a very good thing.

Studying medicine didn't necessarily make you a good doctor,

for one very good reason.

Medical teaching was still largely based on ideas from antiquity.

The disruption of his studies gave Willis the opportunity to investigate the body for himself.

By now, people were exploring the anatomy of the brain.

But still, no-one really knew what it did.

In the mid-1600s, Willis began a ground-breaking series of dissections,

and I'm about to get a privileged glimpse of what he would have seen.

-Ah!

-There we are.

-Human brain. Isn't it wonderful?

-It is.

It is utterly unbelievable when you think that this brain once thought, it reasoned.

It's a unique feature of the universe, really.

When a brain is sort of fresh it's a very different consistency.

Yes, it is, it's... I tell students it's a bit like a badly set jelly.

But presumably if you were to cut that you really would have great difficulties.

Yes, it would just fall to pieces, really.

Willis was one of the first o use a new technique -

preserving brains in alcohol.

This made them firm enough to dissect with great precision.

-You ready to cut this?

-Yes, ready to cut.

Isn't it strange?

Ah!

What's really curious is that there's almost no structure or definition to it, is there?

The thing that really catches your eye is the ventricles in the centre,

which were what everybody was preoccupied with before Willis.

And the idea was that this part of the brain may have acted as a sort of pump,

and important activities may have gone on in the fluid that was moving around in the ventricles.

So in a sense all this is just muscle, and all the thought

and the important stuff is taking place in these holes over here?

Yes, and it was Thomas Willis who realised that

the actual structure of the brain was what was critically important.

When Willis looked at animal brains, he concluded our intellect

and thoughts must lie in the parts of the brain animals don't possess.

Thomas Willis was very struck by the corrugated surface of the human brain

as compared to the smooth surface of the sheep, and this enables

a huge volume of cerebral cortex to be contained

within the relatively small volume of the skull.

-And that's where he thought being human resided?

-Yes.

You can see there's a ribbon of cortex going over the surface

-of the cerebral hemispheres.

-Oh, just there.

-Yes, that's right.

And this cortex was where he realised people were likely to have their thoughts.

Willis had established a link between the state of the brain and the state of the mind.

He wrote the first book specifically about the brain.

From now on, anatomical studies would become

one of the great foundations of a scientific explanation of who we are.

Reason was now seen as the pinnacle of human nature.

It had been shaped by philosophical doubt,

and detailed dissections of the brain.

Europe entered a new age, a celebration of the rational mind.

Faith in reason would underpin the growth of trade

and the building of empires.

In 1837, something was causing a stir at London Zoo.

Their first orang-utan, Jenny,

was introduced to an astonished audience.

Exotic animals were being brought to Britain from across the Empire.

Even Queen Victoria herself came calling.

Jenny's arrival would challenge assumptions about what makes us human.

Right, come this way, Michael, I'll introduce you to Batu,

who should be waiting. There he is.

-There he is. Hello.

-This is Batu.

-Wow, he's big.

-Hello.

-Batu's very big.

-What a beautiful face.

-Very big and very strong.

-Right. Can I do this?

-Yeah, just be careful with the orange.

Yeah.

Oop, very delicately done!

He doesn't want to drop it.

He's even ruder than my kids!

That's rude, stop it.

You could actually see a wonderfully sort of sullen look on his face.

-Yeah.

-That look of "Mm, don't like that."

It's a very human expression.

Odd behaviour. Oh, no, that's terrible!

Ah! Ugh!

It's wonderful, this is a, a great sense of independence.

Stop it now.

You've spat at me.

You've played your game.

What are you going to do next?

Oh, that's smelly!

One of the visitors to the zoo was young Charles Darwin.

But this isn't the familiar story about evolution.

His visit to the zoo was part of his lesser-known research -

fascination with animal emotion.

One day, Darwin saw something that really astonished him.

Jenny was playing with the keeper, and the keeper had an apple,

and the keeper was taunting Jenny by waving the apple in front of her but not letting her get hold of it.

And in Darwin's words, "The ape threw herself on her back and cried precisely like a little child."

Darwin became convinced that the expressions of emotion

he saw in Jenny and in humans were the same.

His research developed over 30 years.

Tenderness, shame, joy -

he saw them all in animals.

Darwin's painstaking work led to one of his most important books,

The Expression Of The Emotions In Man And Animals.

It was greeted with alarm and fascination.

Now this is a really incredible book,

partly because of the illustrations, because this is one of the first books ever to include photographs.

And they feature people, people in various states of distress, if you like.

Disconsolate, sad, very sad-looking.

He examines it in almost microscopic detail.

There's a very interesting picture here of a woman's forehead,

and he notices these two lines coming up here,

which were later called in fact the Darwin grief muscle.

What Darwin was undermining in his work was a fundamental belief -

a belief in human uniqueness.

By suggesting a close kinship with animals,

he'd also opened the lid on the rational mind,

hinting at a dark subterranean world of instincts, desires, emotions -

the animal within.

Here was an irony for Victorian science.

The power of reason, which made us unique,

had been turned on ourselves,

and revealed us to be less exalted, less rational,

than had been suspected.

A new side of ourselves was being unearthed,

darker and more dangerous.

In Paris, doctors began to explore this untamed side,

at La Salpetriere.

This imposing-looking building was originally used to store gunpowder,

but then they decided they could put it to better use,

to lock away thousands of people who were regarded as just as

unstable and dangerous - the destitute and the insane.

It had been Europe's most notorious women's asylum,

with nothing to offer but cruel imprisonment.

These are some of the cells where they kept the women,

and these are the original bars behind which they were imprisoned.

And there is something terribly poignant about the idea of thousands of women chained up,

in filthy living conditions,

utterly without any prospect of release, no hope, no hope at all.

But attitudes were changing.

After years of revolution, the asylum had become a place of care

rather than simply imprisonment.

One of its most famous physicians was Jean-Martin Charcot.

Often the best way to understand the normal is to study the abnormal,

and here there were 5,000 troubled minds to study.

Charcot was one of the first people to try and separate out

and categorise different forms of mental and neurological illness.

He took incredibly detailed notes, and he also took lots of photographs.

One condition in particular had been puzzling doctors.

They called it hysteria.

Patients suffered paralysis, seizures, blindness, and violent fits.

Charcot presumed these symptoms were caused by a physical disease,

but then he began to use a remarkable new approach.

Five, six...

Hypnosis.

..Seven...

Charcot found he could induce and relieve

symptoms of hysteria using hypnosis.

And become aware of any feelings of lightness, going up.

It could produce extraordinary effects in the body.

Drifting up and up now, and the balloon really sort of taking off now and bobbing from side to side.

OK, can you see the balloon?

I can, it's a big blue balloon.

-OK, and it's...

-A sort of Winnie the Pooh blue balloon.

OK. Well, you get that feeling of the...

'I've tried hypnosis before, but this is the first time it's really worked.'

OK and just notice what's happening there.

'Over the course of an hour, I mysteriously lost co-ordination of my hand.'

And that's even more noticeable in fact, it's becoming really shaky now.

'I had my hands stuck together.'

Knuckles are quite locked.

-Oh!

-They are quite locked.

'And most bizarre at all, one side of my visual field was rendered almost useless.'

-Seems a bit fainter.

-OK.

And, um, I have a sense of something in there but not really.

-OK.

-Not really objects.

-OK.

One, two...

'That was extremely odd.'

It was a bit like I was there but I wasn't there,

that he was talking to some other part of me,

and the other part of me was responding.

-Higher, and higher.

-And the idea you can just do it with the power of words...

quite strange.

Charcot's observations of hysteria led him towards

a radical conclusion.

If symptoms could be induced or relieved by hypnosis,

then perhaps they were not signs of some pathological disease.

Perhaps they were caused by emotions,

that the patients themselves were not even aware they were feeling.

Charcot never fully grasped what he was dealing with,

what we would now call the unconscious mind.

In amongst the crowds at one of Charcot's famous demonstrations

was a young Austrian doctor, Sigmund Freud,

a man who would famously use the study of hidden emotions

and repressed urges to develop this extraordinary concept

of the unconscious mind.

Freud's ideas would become a significant cultural influence

on the 20th century.

They would join a rising tide of other ideas

that would form a wholly new approach to who we are - psychology.

A less than rational self had been revealed -

by animals brought back from distant lands,

by changing attitudes to mental illness,

and a new door into the unconscious mind.

We could no longer see ourselves simply as creatures of reason.

By the end of the 19th century,

Europe was in the throes of a bold new age of communication.

Thousands of miles of new railway linked the continent's great cities.

Telegraph cables joined people across the globe.

This interconnected world

led to a different way of looking at how the brain works.

This new technology, naturally enough,

inspired new metaphors to describe the nervous system.

For example, if I pinch my finger, then the pain fibres go

down the line, up into my spinal cord and from there to the brain.

The thing is, what happens next?

Well, everyone knew there were complicated signal boxes and junctions up there,

but nobody knew just how they worked.

The Spanish countryside.

Home to a scientist I deeply admire.

He had a passion for art that would shape his future career as a neuroscientist.

His name was Santiago Ramon y Cajal.

When he was a young man, Cajal was obsessed by art.

As he later wrote, "I was gripped by an irresistible mania.

"I painted everything that captivated my sight - earth, foliage, plants, the human form".

He was actually extremely good at putting down on paper what he saw.

Cajal's passion for art

was coupled with a fascination for a new technology - photography.

This is the sort of camera that Cajal would have used.

I've got it lined up on the mountains now.

I've got a photographic plate in here,

which is basically a bit of glass with some photosensitive chemicals on.

And then you lift this.

And you trigger the shutter.

It should take about 20 seconds.

When that's done, this goes down,

and the glass plate you take away with you

off to the mysteries of the darkroom.

It was his twin passions, art and photography, that would shape

his most important discovery - what it is that makes the brain work.

To see, observe, and make things visible

is one of the great challenges of science.

The challenge for neuroscientists

was uncovering the fine structure of the brain.

The task Cajal set himself was to reveal the communication networks

that exist inside our heads.

I've come to the Cajal Institute to see how he did it.

I always feel like I'm getting into surgery again. Great.

-So...mouse?

-Yeah, take the brain.

'My first job is to chop up a rather slippery mouse brain.'

Very small. Hey!

'It's trickier than it looks'.

There we go.

-Feels like cutting onions.

-Yes!

I'm good at cutting onions.

'The search was on for a stain that would make the mysterious

'structure of the brain visible under the microscope.'

'Cajal was shown a technique using chemicals from the darkroom,

'chemicals that could make brain tissue turn black'

You can see it's a really complicated process,

lots of different stages.

Cajal spent nearly 20 years fiddling away,

doing minor adjustments, just seeking perfection.

The great debate was whether the brain was just a mesh of fibres,

or made of distinct individual units.

Placing stained tissue under the microscope, Cajal became convinced

that there were individual building blocks in the brain - neurons.

Now, that is absolutely beautiful.

That is a neuron.

That is what they were looking for.

Now, the signal goes up here into the cell body,

and then somehow gets distributed by thousands of axons and dendrites,

which link in with all the other neurons in the brain.

Now, only about 1 in 40 of the neurons actually get stained,

and that might sound like a bad thing, but it's actually an incredibly good thing

because if all the neurons here were stained,

then this would be a confusing mass.

You wouldn't be able to make any sense at all.

But because it's just 1 in 40, you can pick them out.

You can see Cajal's artistic influence here -

beautiful drawings of neurons.

He mapped out groups of neurons,

and theorised how they might work -

that nerve impulses travel along them in one direction,

passing from one cell to the next.

Many years later, his theories would be confirmed.

Cajal realised that these neurons are the basic units of the human brain.

We now know there are at least a hundred billion of them,

and all these connecting branches, well, there are trillions of connections.

And somewhere in here, emotion and thought are born.

Somewhere in here is the answer to what makes a human.

Half a century later, the world descended into chaos.

Out of the turmoil of World War II came a secret invention,

built here at Bletchley Park in rural England.

Colossus - the most complex machine that had yet been built.

Designed to crack enemy codes,

it would also shed light on the question of who we are.

What was truly astonishing about Colossus

was the speed at which it could work.

Enemy messages which had previously taken teams of human code-breakers

six weeks to crack could now be done by the machine in six hours.

It must have seemed truly superhuman.

Here was a machine doing something that till now

only the intelligent human mind could do, but much faster.

Once again, the technology of the day

offered a model for how the brain might work.

When you think about it, it's a bit like a primitive brain,

with the valves representing the neurons

and the wiring representing the connecting axons and dendrites.

People had begun to theorise that Cajal's neurons

worked a bit like electronic switches.

If intelligence could be replicated by the on-off switching of a machine,

perhaps the reasoning mind wasn't as uniquely human as we thought.

One of the biggest human brains at Bletchley was Alan Turing,

often called the father of modern computing.

In 1950, he thought of an ingenious way of judging whether computers

show some form of intelligence, by devising a test.

The Turing test is actually more of a Turing question.

The question he asked himself was,

would it be possible to build a computer that was so intelligent

and so good at having chats with humans

that you could be chatting to the machine

and not be aware that you're not actually talking to another person?

Well, he suggested that by the year 2000, we would have cracked the problem.

We are well beyond that point. Let's see.

Right, "what is your name?"

You don't remember? No, I don't remember.

'I'm plugged into one of the more sophisticated programs,

'designed to respond to Turing's challenge.'

OK, let's try some, er, general knowledge.

I mean, computers should be able to do general knowledge.

'It doesn't ever seem to really answer the question.'

Anyway, this is garbage.

'Let's try a different tack - favourite films.'

Transformers 2.

Maybe that is some sort of computer joke.

I can't believe anybody liked Transformers 2.

"What films make you cry?"

"Science fiction and comedy. What do you like?"

Right. It's not very impressive.

I'm not enjoying myself. I'm not having a great conversation here.

I think what you can learn from this is that computers are good at computing,

basically, crunching numbers and things like that.

What they clearly lack is the thing that

really gives any form of human interchange any worth, any value -

feelings like humour, warmth, love, affection,

any of the things that we actually value.

Perhaps too much to expect from a machine.

-Bye bye.

-ELECTRONIC VOICE: Goodbye. Goodbye.

For centuries, technology has provided metaphors to explain who we are.

The computer is simply the latest we have seized on.

But its failings reveal that what makes us human

lies in something a machine cannot do.

We are passionate, irrational creatures,

often driven by forces we do not understand.

At the turn of the 20th century, a great nation was coming of age.

The United States.

The land of the free, personal rights and liberties.

This was the perfect home for the thriving discipline

that focused on ourselves as individuals - psychology.

Psychology, as the name implies, originally started out as

the study of the psyche, or mind.

The idea was, you could look into yourself, introspect,

and learn about human nature that way.

However, here in America, a small group of psychologists soon decided

that was nowhere near rigorous or vigorous enough.

They wanted to turn psychology into a science, so they decided to

focus on something they really could measure and manipulate - behaviour.

This approach, called behaviourism,

was transformed into a systematic science

by one of the 20th century's most controversial pioneers.

His name was BF Skinner.

Skinner was convinced that our behaviour

is the product of our environment,

learnt from our experiences.

Since Skinner thought that environment was all-important,

I thought it would be quite interesting to have a look at where he worked.

This is his study. Isn't it wonderful?

This is completely unchanged from when he died, over 20 years ago.

He liked music, so he had this adapted

so that he could just pull that,

and play his music.

This is a man who likes to tinker and adjust things.

This is the bed in which he used to sleep.

It is absolutely filled with his paraphernalia.

It was his passion for gadgets,

for things that he could adapt and change,

that led him to his greatest invention,

a device which is as iconic to behaviourists

as the telescope is to astronomers -

the operant conditioning chamber, or Skinner's box.

Skinner's experiments would reveal something surprising,

and very disturbing, about the human condition.

-This is an operant chamber.

-Otherwise known as a Skinner box.

It's a Skinner box. Many people in my field...

'Dr Robert Allan uses similar methods to those Skinner used.'

Here's an area where the pigeon stands.

Their response keys...

'The pigeon has to peck on these buttons.

'If it pecks them in the right order, it gets a reward.'

So what are you going to do to impress me with the pigeon today?

I'll show you. Let's go get a pigeon.

Who's this?

This is G21.

G21? I don't think of pigeons as being smart, I must admit.

-They're very smart.

-Is he going to demonstrate just how smart?

-Indeed.

-OK. In you go, G21.

-OK.

-Ooh. Is he hungry?

-It looks like!

'The pigeon has to work out whether the centre light

'shines red or green for longest.

'If it's green, it has to peck the button on the right.'

Oh, he's smart.

Long green means go right.

OK. So will he go right?

-Yes, he will.

-You're confident in your bird, aren't you?

-I am very confident.

-Ah! Very good.

-There you go.

If it was red that was longest, he has to go the other way.

Now he has to go left.

-OK.

-Watch.

Yes, he's done it. He's very good, I have to say.

I'm good at predicting behaviour.

Well done, G21. Go, boy, go.

'What these experiments showed was how easily behaviour could be learned, even manipulated.'

'I was about to see how quickly this can happen.'

We are going to shape the turning response

by delivering reinforcers for his approximate behaviour.

-You're going to make him sort of turn in a circle, are you?

-That's correct. That's better said!

'Each time the pigeon turns left,

'Dr Allan delivers food to reinforce that behaviour,

'until after just 20 minutes,

'he has the pigeon dancing round in circles.'

'Pigeons and birdseed may not look controversial,

'but what was so shocking at the time

'was that Skinner applied his ideas to human behaviour.'

What Skinner was saying is that we are in many ways like pigeons -

that we are the product of the numerous interactions we have with our environment,

whether it's falling in love, the job, the friends you make,

all these things which appear to be decisions are actually

the product of things that have happened to us in the past.

We can no more exercise free will than this pigeon

can decide whether to peck, or indeed, turn in a circle.

Skinner was convinced his discovery could be used to benefit mankind.

We could change people's behaviour for the better by changing their environment.

But in the context of the Cold War, the ability to control behaviour

left some people fearful it could be misused,

because in Skinner's view, free will was nothing but an illusion.

Now, most of us believe that being able to make choices is an important part of being human,

but here was Skinner saying that that was an illusion,

that actually it was a piece of pre-scientific nonsense,

akin to believing in a flat Earth or demonic possession.

You can imagine how popular that message was in the land of the free and the rugged individual.

Behaviourism was soon joined by other approaches,

through the 1960s and beyond.

There were new drugs, therapies, personality tests,

new ways to measure our thoughts, memories and emotions.

Psychology has grown into a vast science,

as diverse and multi-faceted as we are.

So, who are we?

Well, we are the product of our genes and our environment.

Billions of neurochemical reactions

firing every single second of our lives.

In us, reason and emotion are frequently at war.

Thoughts, passions, memories and behaviour

emerge unbidden out of the depths.

Brain scans reveal many parts of the brain

operating outside our conscious awareness.

We are the product of numerous daily interactions,

and the quest to understand the essence of who we are

has revealed something fascinating going on inside our heads,

something none of us are ever aware of.

I can show you what I mean with a famous visual illusion.

HE LAUGHS

'It's called the Ames room.'

That is so bizarre!

Clearly, what I'm seeing is,

I'm seeing a very, very tall person over there

and a short person over there, and when they swap over,

there's a moment when my brain just goes clunk.

I absolutely know this is an illusion, but my brain just won't let me see through the illusion.

So how's it done?

Well, if you come over this way,

it's really obvious.

Hi, there. Thank you.

OK, so essentially,

the room really dips downhill. Lots of space above my head.

There is a sharply sloping floor.

As I march up, the room begins to narrow until I'm really crunched into the corner.

There's very little space between the ground and the top here,

and that's how the illusion is created.

Essentially, the room is a trapezoid.

The Ames room shows us something very important

about how the brain is working.

There's part of my brain which knows the rules of a room.

It has assumptions, models built in there,

and it knows, based on experience,

that normally in rooms, the ceiling and the floor is parallel,

and that the walls are at a right angle.

From one particular viewpoint,

the room looks like it fits that mental model,

and the brain has such a powerful belief that this quirky-shaped room

is normal that people appear to have changed size.

This illusion reveals something fundamental about how the brain works.

Our perception of reality is not just based on what is out there,

but it is also partially constructed.

We have these models running in our head,

and they are constantly being tested against the evidence of our senses.

This process of building models in our heads is happening from the moment we are born.

This child is using her senses to find out about the world.

Is that person in the mirror another baby, or is it me?

Why does that thing make a noise when I shake it?

What she's doing is constantly learning

by testing everything around her.

Thousands of little experiments like these

will create her unconscious assumptions.

They'll build the models that shape her view of the world.

That's how she will be able to make her way through life.

It is very charming when you think that in a way, what she's doing now

is acting rather like a mini-scientist.

She's investigating the world, she's forming her theories,

her hypotheses, and she's testing them against reality.

'And that, in a sense, is what science is, and it's going on

'inside each and every one of us from the moment we're born.'

Is that right, Chloe? Is that right?

It is.

In this programme, we've seen that humans are creatures of both rational thought,

and emotional turmoil.

And in this series, I've shown how science too

has been shaped by reason and emotion,

as well as by the tumult of the world in which it operates.

Its intellectual achievements have transformed our lives.

But it hasn't been straightforward.

The story of science is a messy one,

wrapped up in politics, belief, money and rivalry,

proof forever shaped by power and passion.

Science is a very human activity,

something we unconsciously do every day -

observing the world, building mental models, and testing them.

But it's when we deliberately started using the scientific method

that we went way beyond our individual capabilities.

I think science is the greatest collective endeavour

that mankind has ever undertaken.

Over the last few thousand years,

the human brain has not changed at all.

Evolution does not go that fast.

But what has changed is our understanding of the world.

We don't have to rely simply on the wisdom of our own brains.

SHE GURGLES

We have language, we have literature,

and now we have computers, and that links us all together.

That gives us, if you like,

the wisdom of all those who have gone before.

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