Those of us who love algebra (and even those who don’t) are familiar with problems that end up with the command to ‘solve for X’. But why, of all the letters in the alphabet, has X been chosen for this singular honor of representing the unknown, a practice that has extended well beyond algebra? To the extent that any of us thought of this at all, we may have put it down to sheer accident. Someone back in time picked that letter for who knows what reason and it stuck as others followed the practice.
But it turns out that there is actually a reason for this and on the NPR show Ted Radio Hour, host Guy Raz talked with Terry Moore about it. Moore says that the explanation lies in the roots of modern mathematics that can be traced back to the first couple of centuries in the Common Era.
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.
Moore says that in the original Arabic, the word for the unknown was ‘sheiun’ which meant ‘something’ or ‘some undefined thing’ and that this word was represented by the Arabic letter for the sound ‘sh’. As the importance of these mathematical techniques became more widely realized, these early Arabic texts made their way to Europe via Spain in the 11th and 12th centuries and needed to be translated and this is when the first Spanish translators encountered a problem.
The problem for the medieval Spanish scholars who were tasked with translating this material is that the word sheiun can’t be rendered into Spanish because Spanish doesn’t have that S-H – that sh sound. So by convention, they created a rule in which they borrowed the C-K sound – the cuh sound – from the classical Greek in the form of the letter Chi [Χ]. Later, when this material was translated into a common European language – which is to say Latin – they simply replaced the Greek Chi 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.
You can watch Moore’s Ted talk.
You can also listen to Moore’s interview with Raz.