" Muslim scholars have identified four essential qualities in human beings, which have been identified in earlier traditions as well. Imam al-Ghazālī and Fakhruddīn al-Rāzī adopted them, as did Imam Rāghib al-Isfahānī in his book on ethics. According to Imam al-Ghazālī, the first of them is quwwat al-ʿilm, known in Western tradition as the rational soul, which is human capacity to learn. The next one, quwwat al-ghaḍab, which may be called the irascible soul, is the capacity that relates to human emotion and anger. The third element, quwwat alshahwah, known as the concupiscent soul, is related to appetite and desire. The fourth power, quwwat al-ʿadl, harmonizes the previous three powers and keeps them in balance so that no one capacity overtakes and suppresses the others. "

― Hamza Yusuf , Purification of the Heart: Signs, Symptoms and Cures of the Spiritual Diseases of the Heart