Medieval Iberia was a unique crossroads of religious, philosophical, mathematical, and scientific speculation. Among its most striking features was an attentiveness to the rich symbolism of numbers and the letters of the alphabet. In the Islamic and Jewish traditions, two towering corpora stand out in this regard: the mystical-pietist literature of the Andalusian Sufi Ibn al-ʿArabī (1165-1240), written in Arabic, and Sefer ha-Zohar ("The Book of Splendour"), the foundational text of Castilian theosophical Kabbalah (ca. 1280s, widely attributed to Moses de León, ca.1240-1305), composed mostly in Aramaic. Despite their temporal, geographical, and philological proximity, no systematic comparison exists of how Sufism and Kabbalah approach letters and numbers. This study proposes "the science of letters" as a comparative methodology and heuristic for such analysis. Moving beyond questions of direct influence, it shows that both Ibn al-ʿArabī and the Zohar employ structurally analogous, hermeneutical approaches to letters, numbers, and geometry as the building blocks of the cosmos. At the heart of this shared paradigm is a geometric vision of the cosmos as a circle emanating from a divine central point, associated with specific letters, numbers, and elements. These convergences challenge the hermetic discursive isolation of Sufism and Kabbalah, reframing their alphanumeric speculations as expressions of a common symbolic grammar and metaphysical paradigm that transcended linguistic and religious boundaries.