The contemporary academic study of religion has its roots in conceptual and theoretical structures developed in the early to mid-20th century. A particularly important example of such a structure is the concept of the “numinous” developed by the theologian and comparativist Rudolf Otto (1869–1397) in his work, The Idea of the Holy: An Inquiry into the Non-Rational Factor in the Idea of the Divine and its Relation to the Rational (1923). Building on the work of Immanuel Kant (1724–1804), Friedrich Schleiermacher (1772–1834), and Jakob Fries (1773–1843), Otto developed the concept of the numinous—a “category of value” and a “state of mind”—as a way to express what he viewed as the “non-rational” aspects of the holy or sacred that are foundational to religious experience in particular and the lived religious life in general. For Otto, the numinous can be understood to be the experience of a mysterious terror and awe (Mysterium tremendum et fascinans) and majesty (Majestas) in the presence of that which is “entirely other” (das ganz Andere) and thus incapable of being expressed directly through human language and other media. Otto conceives of the concept of the numinous as a derivative of the Latin numen, meaning “spirit,” etymologically derived from the concept of divine will and represented by a “nodding” of the head. Otto argues that understanding the numinous in a satisfactory way requires a scholar to draw upon their own experience of religious sentiments, given its non-discursive and direct nature; this becomes a point of contention among later secular scholars of religion. In later works, such as Mysticism East and West: A Comparative Analysis of the Nature of Mysticism (1932), Otto gives numerous examples of the ways in which the concept of the numinous can be applied cross-culturally to traditions beyond Christianity, such as Hinduism and Buddhism. Otto’s theories regarding the numinous have been extremely influential in the development of the academic study of religion in the 20th and 21st centuries, as evidenced by the impact they had upon scholars such as Carl Jung, Mircea Eliade, and Ninian Smart, whose works were instrumental in the formation of religious studies as a discipline. Jung cites the concept of the numinous extensively with regard to his theories on the breakthrough of unconscious material into conscious awareness. Eliade’s work The Sacred and Profane: The Nature of Religion (1959) takes Otto’s concept of the numinous as a starting point in the development of its own theory; Eliade’s use of the category of the “sacred” might be considered derivative of Otto’s larger conception of the “holy” (das Heilige). Eliade’s work, like Otto’s, has been extensively criticized for postulating a sui generis nature of both the numinous and the sacred, which are viewed by Eliade as irreducible to other phenomena (historical, political, psychological, and so forth). Smart’s influential “dimensional analysis” theory and his scholarship on the topic of world religions is highly informed by his utilization of Otto’s theory of the numinous within the contexts of his cross-cultural reflections on religion and the development of his “two-pole” theory of religious experience. The concept of the numinous continues to be theorized about and applied in contemporary academic research in religious studies and utilized as part of a framework for understanding religion in university courses on world religions and other topics in the academic study of religion. In part through the work of Eliade, Smart, and other scholars—Otto included—who have found a popular readership, the term has been disseminated to such a degree as to find common usage in the English language and popular discourse.