Seriously. It's a pretty easy question. You should be able to answer it.

But how do you know? How does

anyone know anything? You might say, well, I know where my fingers are. I'm looking

right at them.

Or, I can touch them, I can feel them, they're right here and that's good.

Your senses are a great way to learn things.

In fact, we have way more than the usual five senses we talk about.

For instance, your kinesthetic sense, proprioception.

This is what the police evaluate during a field sobriety test.

It allows you to tell where your fingers and arms and head and legs in your body

is all in relation to each other

without having to look or touch other things.

We have way more than five senses, we have at least twice as many

and then some. But they're not perfect. There are optical illusions,

audio illusions,

temperature sensation illusions, even tactile

illusions. Can you turn your tongue upside down?

If so, perfect. Try this. Run your finger

along the outer edge of the tip of your upside down

tongue. Your tongue will be able to feel your finger, but in the wrong place.

Our brains never needed to develop an understanding

of upside down tongue touch. So, when you touch the right side of your tongue

when it's flipped over to your left side you perceive a sensation

on the opposite side, where your tongue usually is but isn't

when it's upside down. It's pretty freaky and cool

and a little humbling, because it shows the limits of the

accuracy of our senses, the only tools we have to get what's out there

in here. The philosophy of knowledge,

the study of knowing, is called Epistomology.

Plato famously said that the things we know

are things that are true, that we believe

and that we have justification for believing. those justifications might be

irrational

or they might be rational, they might be based on proof,

but don't get too confident because proven is not a synonym

for true. Luckily, there are things that we can know

without needing proof, without needing to even leave the house, things that we can

know as true

by reason alone. These are things that we know

a priori. An example would be the statement

"all bachelors are unmarried." I don't have to go survey

every bachelor on earth to know that that is true.

All bachelors are unmarried because that's how we define

the word bachelor. Of course, you have to know what the words

bachelor and unmarried mean in the first place.

Oh, you do? Okay. Perfect. That's great. But

how do you know? This time I mean

functionally, how do you know? Where is knowledge biologically

in the brain? What are memories made out of?

We are a long way from being able to answer that question completely

but research has shown that memories don't exist in the brain in single

locations. Instead, what we call

a memory is likely made up of many

different complex relationships all over the brain between lots of brain cells,

neurones. A major cellular mechanism thought to underlie the formation of

memories

is long-term potentiation or

LTP. When one neurone stimulates another neurone

repeatedly that signal can be enhanced overtime

LTP, wiring them more strongly together

and that connection can last a long time, even

an entire lifetime. A collection of different brain cells,

neurones that fire together in a particular order

over and over again frequently and repeatedly

can achieve long-term potentiation, becoming

more sensitive to each other and more ready to fire in the exact same way

later on in the future. They're a physical thing

in your brain, firing together more easily

because you strengthen that pattern of firing.

You memorized. This branching forest of firing friends

looks messy, but look closer. It could be the memory

of your first kiss. A living souvenir

of the event. If I were to go into your brain and cut out

those cells, could I make you forget your first kiss

or could I make you forget where your fingers are?

Only if I cut out a lot of your brain.

Because memories aren't just stored in one relationship, they're stored

all over the brain. The events leading up to your first kiss are stored in one

network,

the way it felt to the way it smelled in different networks, all added up together

making what you call the memory of your first kiss.

How many memories can you fit

inside your head? What is the storage capacity

of the human brain? The best we can do is a rough

estimate, but given the number of neurons in the brain involved with

memory

and the number of different connections a single neurone can make

Paul Reber at Northwestern University estimated

that we can store the digital equivalent of about 2.5

petabytes of information. That's the equivalent of recording a TV channel

continuously for 300 years.

That's a lot of information. That is a lot of information about

skills you can do and facts and people you've met,

things in the real world.

The world is real, right?

How do you know? It's a difficult question,

but it's not rocket science. Instead, it is

asking whether or not rocket scientists even exist in the first place.

The theory that the Sun moved around the earth

worked great. It predicted that the Sun would rise every morning

and it did. It wasn't until later that we realized what we thought was true

might not be. So, do we

or will we ever know true reality

or are we stuck in a world where the best we can do is be

approximately true? Discovering more and more useful theories every day but never

actually reaching

true objective actual reality.

Can science or reason ever prove convincingly

that your friends and YouTube videos and your fingers

actually exist beyond your mind? That you don't just live

in the matrix?

No. Your mind is all that you have,

even if you use instruments, like a telescope

or particle accelerators. The final stop

for all of that information is ultimately

you. You are alone in your own brain,

which technically makes it impossible to prove that

anything else exists. It's called

the egocentric predicament. Everything you know

about the world out there depends on

and is created inside your brain.

This mattered so much to Charles Sanders Peirce that he drew a line

between reality, the way the universe truly is,

and what he called the phaneron, the world as filtered through our senses

and bodies, the only information we can get.

If you want to speak with certainty you live in,

that is you react to and remember and experience your

phaneron, not reality.

The belief that only you exist

and everything else, food, the universe, your friends are all

figments of your mind is called solipsism.

There is no way to convince

a solipsist that the outside world

is real. And there is no way to convince someone who

doubts that the universe wasn't created just three seconds ago

along with all of our memories. It's a frightening realization

that we don't always know how to deal with. There's even

The Matrix defense. In 2002

Tonda Lynn Ansley shot and killed her landlady.

She argued that she believed she was in the matrix, that her crimes

weren't real. By using the matrix defense

she was found not guilty

by reason of insanity, because the opposite view is just way more healthy

and common.

It's called realism. Realism is the belief that the

outside world exists independently of your own

phaneron. Rocks and stars and Thora Birch

would continue to exist even if you weren't around

to experience them. But you cannot

know realism is true. All you can do

is believe. Martin Gardner, a great source for math magic tricks,

explained that he is not a solipsist because realism is just way more

convenient

and healthy and it works.

As to whether it bothered him that he could never know realism was true,

he wrote "if you ask me to tell you anything about the nature of what lies

beyond the phaneron,

my answer is how should I know? I'm not dismayed by ultimate mysteries,

I can no more grasp what is behind such questions

as my cat can understand what is behind the clatter i make

while I type this paragraph." Humble stuff.

What strikes me

is the cat. Cats do not understand keyboards,

but they know the keyboards are a fun place to be.

It's a great way to get the attention of a human, they're warm

and exciting, surrounded by noises and flashing lights plus

cats love to get their scent on whatever they can,

a mark of their existence.

We aren't that much different, except instead of keyboards

we have the mysteries of the universe.

We will never be able to understand all of them. We won't be able to ever answer

every single question,

but walking around in those questions, exploring them,

is fun. It feels good.

And as always,

thanks for watching.

Do you want more unanswered questions? Well, you're in luck.

Today, nine other amazing channels on YouTube have made videos

about questions we still haven't fully answered.

Alltime10s has organized them and to watch them all

click the annotation at the end of this video or the link

at the top of the description. Enjoy.