Leiden (Netherlands), 1637. The René Descartes Method Discourse was the first edition of one of the fundamental works of Western philosophy that was printed in Jan Maire's printing press.

The work had a section called Geometry. The section title clearly indicates that the French philosopher introduced numerous equations on these pages, as the book supervisor notes. And that caused a lot of headaches to the printers. At that time, the embossed typographic letters were composed of words, lines and, therefore, pages. Therefore, they needed more specimens of the most commonly used types. And the need was heightened when you wanted to print the equations with the text. The first letters of the alphabet were used to represent the strangers of the equations: a, b, c... The letters were one of the most used in the texts.

To solve the shortage of the most common letters, they asked Descartes whether it was necessary to use the first letters of the alphabet in the equations or whether any letter could be used. He answered that in the equations the letters are nothing more than symbols, whatever use is made of them. With the permission of the author, the printer decided to use less used letters. As the last letters of the alphabet were within this group, they turned back to the old custom, and instead of using the first letters of the alphabet, they moved on to the last.
Therefore, they have been used since then to represent the strangers of the equations x, y y y z. They say there's no evidence to prove the legend. In addition, there are other theories that state that the use of these letters, that of the X, is older: Amin Mallouf, in the book Samarcanda, points out that his origin is in the mathematician Omar Khayyam, who in the 12th century used the word xhay (unknown) and that it would be the abbreviation of the word X. But Mallouf often confuses historical reality and fiction in his works, while Khayyam did not write his works in the Latin alphabet. That is why some say that the vast mathematical knowledge of the Arabs started using X when it entered Europe and translated the mathematical treaties into European languages, although there is no evidence of it. Florian Cajori says in A History of Mathematical Notations that the theory of Arab origin has no credibility and that the most logical explanation is that of Descartes (and we know the importance of logic in Cartesian philosophy).

However, using the words of Descartes himself, “to investigate the truth, as far as possible, all things have to be questioned.”