Otto's most famous work is The Idea of the Holy, published first in German in 1917, and first translated into English in 1923. It is one of the most successful German theological books of the 20th century, has never gone out of print, and is now available in about 20 languages. The book 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".-Print ed.