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 are 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 epistemology.

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,

neurons. A major cellular mechanism
thought to underlie the formation of memories is long-term potentiation or LTP.
When one neuron stimulates another neuron repeatedly, that signal can be enhanced over time. LTP, wiring them more strongly together

and that connection can last a long time, even an entire lifetime.
A collection of different brain cells, neurons 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 neuron 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 phenomenon, 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. 

Is Anything Real?

Vsauce

Knowing Your Surroundings and Sensory Limitations

Question of knowing finger location and how we know anything.

Senses as a way to learn, exceeding the typical five senses.

Kinesthetic sense (proprioception) and its role in body awareness.

Imperfect senses and examples of sensory illusions.

Upside-down tongue experiment demonstrating sensory limitations.

Introduction to epistemology (study of knowledge).

Plato's definition of knowledge and the distinction between 'proven' and 'true'.

A priori knowledge and the example of 'all bachelors are unmarried'.

Questioning the origin of knowledge and its biological basis in the brain.

Memories not stored in single locations but as complex relationships in the brain.

Long-term potentiation (LTP) and its role in memory formation.

Neurons firing together and strengthening connections.

Memories as physical things in the brain, not stored in one place.

Memories distributed across brain networks.

Storage capacity of the human brain (estimated at 2.5 petabytes).

Questioning the reality of the external world.

The limitations of scientific knowledge in proving reality.

Inability of science to definitively prove the existence of things beyond the mind.

The egocentric predicament and the role of the brain in creating our experience.

Charles Sanders Peirce's distinction between reality and the phaneron.

Living in and reacting to the phaneron, not reality.

Solipsism and the impossibility of proving the external world's existence.

The Matrix defense as an example of questioning reality.

Realism as the belief in an independent external world.

Martin Gardner's pragmatic acceptance of realism.

Acknowledging the limitations of knowledge and embracing ultimate mysteries.

Analogy of cats and keyboards to illustrate our relationship with the universe.

The joy of exploring unanswered questions.

Concluding remarks and promotion of related videos.