The term 'heart' has a primal position in the vocabulary of Muslim religious teachers and in that of the philosophers. The term is used in Islam for the seat of intellectual and emotional life. In this treatise, al-Ghazali unravels its wonders and secrets, achieving his desired end - to lead man to the good life. This is the twenty-first book of the third quarter of al-Ghazali's Ihya Ulum al-Din.