kind of knowledge; for Allah sent down the Quran as “a healing for what is in the hearts, and guidance and mercy for the believers.” Indeed, this knowledge concerns the very foundations of religion: belief in the heart is the foundation of the tongue’s speech, and the heart’s actions are the foundation of the limbs’ actions. The heart is the king of the body, as Abū Hurayra—may Allah be pleased with him—said:

“The heart is a king, and the limbs are its soldiers. When the king is good, its soldiers are good; when the king is corrupt, its soldiers are corrupt.”

In the two Sahīh collections, the Prophet, peace and blessings be upon him, is reported to have said:

“Indeed, there is a piece of flesh in the body: when it is sound, the rest of the body is sound, and when it is corrupt, the rest of the body is corrupt. Indeed, it is the heart.”

Whoever neither knows what rectifies his inward state and what corrupts it, nor seeks to rectify his heart through faith and warding off hypocrisy, is a hypocrite if he outwardly professes Islam. Both the believer and the hypocrite outwardly profess Islam: Islam is outwardly manifest, whereas faith is in the heart. As reported in al-Musnad, the Prophet, peace and blessings be upon him, said:

“Islam is outwardly manifest, and faith is in the heart.”

The statements of the Companions and their successors, and the hadiths and reports concerning this subject, are far, far more numerous than those concerning leasing, pre-emption, menstruation, and ritual purification. Yet this knowledge is outwardly manifest and available, expressed in speech and recorded in books. The more knowledgeable someone is about the affairs of the heart, the more knowledgeable he is about this subject and the meanings of the Quran and hadith. People generally find these things within themselves through direct experience and inward feeling, so they become