This work investigates the fundamental nature of mystical experience by distinguishing between theistic, monistic, and panenhenic varieties of consciousness. R. C. Zaehner, a renowned scholar of Eastern religions and former Spalding Professor of Eastern Religions and Ethics at Oxford, utilizes a rigorous comparative framework to analyze these states. He argues that not all mystical experiences are identical, challenging the perennialist view that all religions lead to the same ultimate reality. By examining primary texts from diverse traditions, he establishes a taxonomy of experience that separates the natural from the supernatural.

Scholars frequently cite this text as a foundational, albeit controversial, critique of the unified theory of mysticism. Readers often note the dense, academic prose and the author's uncompromising stance on the theological differences between Eastern and Western traditions.