There is a letter that is the protagonist of any equationfunction, proportion: the x. Anyone who has ever performed one of these calculations has encountered it. But why was this letter chosen to indicate “the unknown“?
Its history is almost adventurous: it was born from Arabic pronunciation of the word “What”but it could find an affirmation in modern mathematics thanks to a funny and casual anecdote concerning the French philosopher and mathematician Descartes.
The history of mathematics and its language is closely linked to the Arab culture. Our numbers, in fact, although having their origins in India around the 4th century BC, then arrived in Western culture through Arab scholars such as al-Khuwarizmi and his work Of the Golden Numbers. Mathematicians of al-Khuwarizmi’s time referred to what we call “unknowns” of equations like “the thing”meaning “what was to be discovered”. In Arabic, “what” was said shaywhose pronunciation was very similar to the pronunciation of our letter x.
However, when these notions arrived in Europe, the use of the term was not immediate. x within equations and in the Renaissance period the concept of “thing” continued to be used. However, the Arabic pronunciation was not forgotten and reached the ears of the great philosopher and mathematician Rene Descartes who, in his book Geometry In the 1637consecrated the use of both the term unknown in reference to “that of which we do not know the value”, that the letter x as its symbol.
Descartes’ fame has reached our days both for his mathematical merits, think of the Cartesian axes, and for the very famous philosophical phrase «I think therefore I am». It is perhaps from the union of these two that another important intuition of Descartes is born: to define unknown what we – literally – don’t know within an equation.
When in 1637 he is writing the treatise geometry, Descartes decided to use the Initial letters of the alphabet (a, b, c, …) to indicate the known quantitieswhile those finals (z, y, x, …) as a symbol of the unknownsLogically, it would have been correct to use the first letter for the unknown in an equation as z, being the last letter of the alphabet, and continue backwards with y, x, … for the subsequent unknowns. Themore frequent use of the x to the detriment of z And y It is generally attributed – at least this is the most widespread hypothesis – to the fact that Descartes knew the history of the term “thing” and its Arabic pronunciationso he opted for the letter xso close in pronunciation to Arabic shay.
There is aanother hypothesiscertainly more adventurous. It is said that the printer in charge of printing Geometry was short of zand since the x era and it’s a lot less frequent in Frenchproposed to use the latter and Descartes accepted – perhaps always mindful of the Arab heritage.
There letter xhas then acquired fame over time as well outside the mathematical worldalways to indicate something whose nature is unknown. An example is the X-raycalled this way precisely because initially it was not known what they were. x It has also been used in films or TV series to indicate, for example, superheroes with unknown powers (the X-Men) or investigations to be kept secret (the X-Files).
