Francisco de Quevedo:

Aconseja a un amigo, que estaba en buena posesión de nobleza, no trate de calificarse, porque no le descubran lo que no se sabe

The sonnet's subject matter is clearly indicated in its title. Quevedo, with a good deal of irony, suggests that ignorance of one's lineage is better than, quite literally, opening up the "can of worms" of the genealogical investigation that might reveal some sort of problem. Quevedo emphasizes his theme by referring to the myth of Phaëthon.

The most important technique Quevedo uses in this sonnet —and what holds it together— is alusion. There are really two types of allusion to examine here: the fairly conventional mythological references to Phaëthon, and the essential issue being addressed by this sonnet: the question of lineage and blood purity in Golden Age Spain. The latter was, for Quevedo and his immediate audience, an allusion to a contemporary reality that permeated the life of Spanish society at every level. For us, however, it becomes an historical allusion, and in order to deal with this poem we need to have some sense of what it was all about.Having eliminated the last moorish kingdom on the Iberian Peninsula with the Reconquest of Granada in 1492, Spain was finally able to identify herself as a truly "Christian" country. The Jews were expelled from Spain in the same year, and the moriscos were expelled some years later. While many people of Jewish and Moslem ancestry had converted to Christianity and still lived in Spain as New Christians, the Inquisition was constantly on the alert to make sure that these people were not secretly reverting to the practice of their old religions, whether they had converted recently or their ancestors had turned to Christianity many generations before. A series of laws, the Statutes of Pure Blood, were enacted, some even before 1492. They were designed to protect certain institutions and organizations from "racial contamination," and simply prohibited any person with New Christian ancestry from holding certain kinds of jobs, belonging to certain kinds of organizations, and receiving certain kinds of honors.

There was the prevailing notion in Golden Age Spain that the nobility had more to worry about with regard to blood purity than the peasantry, since the upper classes were simply more likely to have intermarried with people of Jewish extraction. Unfortunately, it was often the case that an upper class Spaniard of Quevedo's day might want to join a very prestigious organization (the military orders were a common example) that required an investigation to produce an "ejecutoria," a document that detailed the results of that investigation. A successful investigation (certification of a person's Old Christian heritage) would guarantee social status and prestige. Should, however, such an investigation produce problems, or even questions, the individual involved would be completely discredited and lose all social status.

Quevedo likens the impulse to investigate one's lineage to the story of Phaëton. Phaëthon insisted, to prove to all that he was the son of Apollo, that his father allow him to drive the chariot of the sun. When Phaëthon could not control the chariot and started wreaking havoc on the world below, Apollo was forced to destroy him, striking with a thunderbolt and casting him down to earth —and to his death— in flames. While references to Phaëthon as a warning against impulsiveness or excessive ambition were fairly common in Golden Age poetry, Quevedo's use of the allusion here is even more compelling, since Phaëthon's story is directly related to the question of lineage.

In order to make his point, Quevedo alternates references to genealogical investigations and to Phaëthon. The first quatrain and the first tercet deal with the matter at hand, and contain Quevedo's advice to not undertake such an investigation. The second quatrain and second tercet refer to Phaëthon, and tie his error —and his fate— to the situation of Quevedo's friend.

This is a poem with a clear message. In order to convey that message, Quevedo resorts to fairly straightforward language and syntax. This gives greater impact to the few devices he does use, which are all designed to underscore the theme.