Stéphane Mallarmé’s poem “Sonnet en -x” is one of the most famous examples of modern self-referential texts. However, it does not contain any explicit motif of self-referentiality, such as a text inside the text (mise-en-abyme) or open comments about literature, language or the poem itself. Nor can its self-referentiality be best characterized as implicit or hermetic. In this chapter, a close reading of the poem and a consideration of Mallarmé’s poetic statements show how this specific poem refers to itself, and how it tries to answer the somewhat eccentric question if it really knows what it is talking about. The main aim is to demonstrate that this poem invites the reader to understand self-reflexivity as a multifarious and open question that productively evokes an understanding of the nature of poetic texts, rather than presupposing it.