The Idea of the Holy: An Inquiry into the Non-Rational Factor in the Idea of the Divine and its Relation to the Rational (German: Das Heilige. Über das Irrationale in der Idee des Göttlichen und sein Verhältnis zum Rationalen) is a book by the German theologian and philosopher Rudolf Otto, published in 1917. It argues that the defining element of the holy is the experience of a phenomenon which Otto calls the numinous. The book had a significant influence on religious studies in the 20th century.