Why is 'x' the unknown?

Why is 'x' the symbol for an unknown? In this short and funny talk, Terry Moore gives the surprising answer.

I have the answer to a question that we've all asked.

The question is,

Why is it that the letter X

represents the unknown?

Now I know we learned that in math class,

but now it's everywhere in the culture --

The X prize, the X-Files,

Project X, TEDx.

Where'd that come from?

About six years ago

I decided that I would learn Arabic,

which turns out to be a supremely logical language.

To write a word or a phrase

or a sentence in Arabic

is like crafting an equation,

because every part is extremely precise

and carries a lot of information.

That's one of the reasons

so much of what we've come to think of

as Western science and mathematics and engineering

was really worked out in the first few centuries of the Common Era

by the Persians and the Arabs and the Turks.

This includes the little system in Arabic

called al-jebra.

And al-jebr roughly translates to

"the system for reconciling disparate parts."

Al-jebr finally came into English as algebra.

One example among many.

The Arabic texts containing this mathematical wisdom

finally made their way to Europe --

which is to say Spain --

in the 11th and 12th centuries.

And when they arrived

there was tremendous interest

in translating this wisdom

into a European language.

But there were problems.

One problem

is there are some sounds in Arabic

that just don't make it through a European voice box

without lots of practice.

Trust me on that one.

Also, those very sounds

tend not to be represented

by the characters that are available in European languages.

Here's one of the culprits.

This is the letter sheen,

and it makes the sound we think of as SH -- "sh."

It's also the very first letter

of the word shayun,

which means "something"

just like the the English word "something" --

some undefined, unknown thing.

Now in Arabic,

we can make this definite

by adding the definite article "al."

So this is al-shayun --

the unknown thing.

And this is a word that appears throughout early mathematics,

such as this 10th-century derivation of roots.

The problem for the Medieval Spanish scholars

who were tasked with translating this material

is that the letter sheen and the word shayun

can't be rendered into Spanish

because Spanish doesn't have that SH,

that "sh" sound.

So by convention,

they created a rule in which

they borrowed the CK sound, "ck" sound,

from the classical Greek

in the form of the letter Kai.

Later when this material was translated

into a common European language,

which is to say Latin,

they simply replaced the Greek Kai

with the Latin X.

And once that happened,

once this material was in Latin,

it formed the basis for mathematics textbooks

for almost 600 years.

But now we have the answer to our question.

Why is it that X is the unknown?

X is the unknown

because you can't say "sh" in Spanish.

(Laughter)

And I thought that was worth sharing.

(Applause)