The world we experience isn't just given to us.
The brain is actively generating our experienced worlds
every moment of every day.
If I'm sitting on the beach, and I open my eyes,
I suddenly see before me the sea, the waves,
I can feel the breeze on my face,
I can hear the seagulls in the distance.
It seems like there's this objective world out there,
and it's just pouring itself into my mind through the senses.
Yet, I know that's not what's going on.
The signals that arrive at our eyes and our ears,
the light waves that hit our retinas,
the pressure waves that come into our eardrums,
they don't come with labels on them, like "I'm from a seagull",
or "I'm from the sea", or "I'm blue".
They're just ambiguous signals.
The brain has to make sense of all these sensory signals
and figure out where they came from and what they mean.
That's the process of perception.
So perception isn't just a reading out of the world around us,
it's always a creative act of interpretation
in which the brain is utilising its knowledge about the way the world is,
to make its best guess of what causes the sensory signals,
and that - that's what we consciously perceive.
So the brain has what you might call prior knowledge,
about the structure of the world and about what's out there,
that allows it to interpret the sensory signals that come in.
And sometimes this prior knowledge
is built deep into the structure of our brain,
and it's knowledge that we're not aware of having.
For example, that light tends always to come from above.
And that means that, when we look at shadows,
our brain is using that knowledge
so that we interpret what shadows mean in a particular way.
It's not so much "I'll believe it when I see it",
but if your brain believes it, then you will see it.
Perhaps the real world
just isn't something that's useful for us to see.
We are evolved biological organisms,
our brains have limited capacities,
we see the world as it's most useful to us in order to survive.
Colour is a very, very familiar feature of our visual experience
that gives our lives texture and beauty and meaning.
Does colour exist in the world?
Well not really, no. There are just light waves of different wavelengths,
and out of those wavelengths,
the brain generates an infinite variety of different colours.
And why does it do this? Well, it allows the brain to keep track
of objects as lighting conditions change.
As the artist Cezanne once said,
"colour is the place where the brain and the universe meet".
This process is going to be a little bit different for each of us,
we're all going to experience our own interpretations
of even the same sensory data.
And this brings up this idea of inner perceptual diversity.
Now we're all familiar with the idea of external diversity.
We all recognise that we're different heights,
different skin colours, different shapes,
and we're less familiar with the idea of inner diversity.
One reason is that you can't see my experience.
If you look at the blue sky, the blue that you experience
might not be the same as the blue that I experience,
even though we'll both call it blue.
There's one example of perceptual diversity,
which became very well known a few years ago.
It was this photo of a dress that half the world saw as blue and black,
and half the world saw as white and gold.
These bits are blue, and these bits are black.
Definitely.
The people that saw it as blue and black
were so convinced that what they saw was the way it is
that they couldn't understand
how other people could see it differently.
And the same went for the people that saw it to be white and gold.
Don't you see it?
Part of the reason was that it was a very badly exposed photograph.
So it was very ambiguous,
and the colour that it seemed to be depended on the assumptions
your brain was making about the ambient surrounding light.
Okay, if we see that so differently,
what about the rest of our experience?
How can we assume that in other cases we're experiencing the same thing?
The answer is we probably aren't.
It can almost be used as a lever
to help people better communicate with each other.
Because if you can demonstrate
that even something as apparently straightforward
as opening your eyes and looking at a photograph
can lead to such different beliefs about what's going on,
then we can begin to recognise
that other people may see things differently,
believe things differently.
A lot of challenges in communicating with each other
might be rooted in the fact
that our literal experience of the same world is different.
It's not that these things don't exist - they exist -
but the way in which they exist for me
is completely dependent on my brain.
We do all experience a shared world, a shared reality,
but our way of experiencing it is individual. It's unique.
And it's very precious because of that.