The book explores the depths and highs of the mystical life in the works of Teresa of Avila with a special focus on the relations between the intellect, will and mystical knowledge. As the Interior Castle, the major source for the work, finds its ultimate peak in the union of the soul with its God, so also this book comes to its fullest in the final chapter and concludes that the mystical marriage is a kind of proto-beatific vision: it already is beatitude but not yet fully so. To enable the reader to follow the argumentation, first part of the book is dedicated to a novel interpretation of Teresa's thought which transgresses the usual approach of following Teresa throughout the seven dwelling places. This book presents a fresh and novel interpretation of Teresa's teaching, highlights the common traits between her and Aquinas and offers a chance to peep into the depth of the mystical life.