Hey, Vsauce. Michael here.
Where are your fingers?
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.
