The Idea of the Holy, published by the German theologian and philosopher Rudolf Otto in 1917 as Das Heilige, argues that the essence of religious experience is not primarily moral, rational or doctrinal but a distinct, non-rational category of feeling Otto names the numinous, from the Latin numen, a divine presence or power. Otto argues that earlier accounts of religion had wrongly reduced it to ethics or rational belief, missing the specific quality of experience, prior to and irreducible to moral or conceptual content, that he holds actually generates religious response across traditions.
Otto characterizes the numinous experience through a set of terms that have become standard reference points in religious studies: it is felt as mysterium tremendum et fascinans, a mystery that is simultaneously terrifying and overwhelming, provoking dread and self-abasement before an overwhelming power, and fascinating, drawing the person experiencing it toward continued encounter despite, or because of, that same overwhelming quality. He treats this combination of dread and attraction as a genuinely distinct category of human experience rather than a compound built from ordinary fear and desire.