Rudolf Otto's classic The Idea of the Holy brims with deep theological and philosophical insights into the theory of religion and spiritual belief in God. As both a theologian and a philosopher, Otto was fascinated by how the rational and non-rational interact and interplay with regard to humanity's religious beliefs. This investigation determines that valid, non-rational experiences are as poignant in the development of knowledge - yet how do we quantify and apportion value to various rational arguments for faith, and the experiences so many believers have had over the millennia? The author delves into these arguments, and valiantly attempts to determine how the divine interacts with various emotions. He coins the term 'numinous' from the root word 'numen', to denote an emotional state of consciousness apt to receive spiritual insight. The many emotions and attuning of the soul to God falls under the designation, and it proved to be one of the most important ideas Otto would ever advance. remains an important text for students of both philosophy and religion, as well as spiritually inclined readers. Although Christian believers will profit from its concepts and deep discussion of spiritual matters, those of a different faith or of none often come to respect the intricacy and depth with which Otto explains and qualifies his theology in rational and non-rational terms. This edition contains the text of the original first translation to English of 1923 by John W. Harvey, who strived to preserve the clarity, concision and poise of Otto's lessons. Since the book was originally published in German in 1918, it has been translated into over twenty different languages.