Se =The Unknown Quantity: The Spiritual Function of Sex by Alexander J. McIvor-Tyndall explores sexuality through a spiritual, philosophical, and metaphysical perspective. Rather than treating sex solely as a physical or biological experience, the work considers its broader relationship to human consciousness, personal development, relationships, and spiritual life.

Drawing on the spiritual thought of its period, the book examines ideas about the connection between sexual energy, emotional life, morality, and higher forms of human development. It provides a historical example of early twentieth-century approaches to sexuality and spirituality and will interest readers studying spiritual philosophy, metaphysical thought, sexuality, and the history of alternative approaches to human relationships.