The key point of this work is the consideration of the book of law as an inviolable good. However, I have decided to highlight a well-known topic within the particular phenomenon I am analyzing: the relationship between the multiple and the one. This implies that the philosophical foundation, so to speak, occupies a central place in the medieval Castilian legal discourse that I analyze. In this article, I will discuss how the legal text of the Siete Partidas of King Alfonso X of Castile self-institutes as an inviolable book.