This book traces the modern history of ineffability, the idea that mystical experience lies beyond the reach of language, across twentieth-century scholarship on mysticism. Moving from William James to W.T. Stace and beyond, it examines how theories of ineffability were shaped not only by philosophical commitments but also by rhetorical strategies and disciplinary politics within the academic study of religion. Through close readings of major figures including James, Underhill, Otto, Zaehner, Stace, Smith, Hick, Forman, d’Aquili, and Sells, the book uncovers a crucial historical shift: from early understandings of ineffability as relative, what cannot be fully but can still be partially expressed, to later claims that mystical experience or its source is absolutely ineffable, wholly beyond conceptual mediation. This “absolutization” of ineffability, the book argues, served to defend a perennialist vision of religion’s universal core and to shield it from contextual critique. Concluding with a semiotic “meta-argument” against absolute ineffability, the study proposes that a theory of relative ineffability better accounts for both mystical discourse and ordinary experiences of the unsayable. In doing so, it reorients the study of mysticism toward a more nuanced understanding of how language, experience, and meaning continually shape one another. It is an important read in the study of mysticism.