Rudolf Otto (1869–1937) was a German Protestant theologian and scholar of comparative religion. Otto's most famous work, "The Idea of the Holy" (published first in 1917 as "Das Heilige"), defines the concept of the holy as that which is "numinous". Otto explained the numinous as a "non-rational, non-sensory experience or feeling whose primary and immediate object is outside the self." It is a mystery (Latin: "mysterium tremendum") that is both fascinating ("fascinans") and terrifying at the same time; A mystery that causes trembling and fascination, attempting to explain that inexpressible and perhaps supernatural emotional reaction of wonder drawing us to seemingly ordinary and/or religious experiences of grace. This sense of emotional wonder appears evident at the root of all religious experiences. Through this emotional wonder, we suspend our rational mind for non-rational possibilities.