Upon completing the monumental The Principles of Psychology, William James turned his attention to serious consideration of such important religious and philosophical questions about the nature and existence of God, the immortality of the soul, and free will and determinism. His interest in these subjects found expression in a variety of works, among them, this classic study of spirituality. James based The Varieties of Religious Experience on his participation in the University of Edinburgh's Gifford Lectures on Natural Religion. Studded with concrete examples, it documents and discusses various religious states of consciousness and covers such topics as the meaning of the term "divine," the reality of the unseen, the religion of healthy-mindedness, the sick soul, the divided self and the process of its unification, conversion, saintliness, and mysticism. This work remains one of the great books on the subject, particularly for the evidence it cites for religious experience as a unique phenomenon.