Principle Thirty-Three: Illness in the Quran—namely, the sickness of hearts—appears in two forms: the illness of doubts and misgivings, and the illness of lusts and forbidden desires.

The key to distinguishing one from the other—despite their frequent mention in the Quran—is the surrounding context. If the passage censures the hypocrites or others who oppose some aspect of the religion, the sickness is that of doubts and suspicions. If the verse speaks of sins and illicit inclinations, the sickness is that of base desires.

Why is the malady confined to these two? Because a diseased heart is the opposite of a sound heart, and full heart-health rests on two pillars: perfect knowledge, understanding, and certainty, and perfect will and love for whatever Allah loves and approves.

A sound heart, therefore, is one that recognizes the truth and follows it, and recognizes falsehood and avoids it. When its knowledge is tainted by uncertainty—harboring misgivings that contradict what Allah has revealed of doctrinal foundations and practical rulings—its perception becomes distorted, and the severity of its illness waxes or wanes in proportion to those doubts and suspicions. Should its will and love incline toward any act of disobedience, that very inclination constitutes a corruption of its desire and a sickness.

Both afflictions may converge, leaving the heart deviant in both cognition and volition.

An example of the first type is the Almighty’s words: “In their hearts is a sickness” (2:10)—that is, inherited traditions, doubts, and misgivings that oppose the message of Muhammad, Allah bless him and grant him peace—followed by “so Allah increased their sickness” (2:10), a punishment for a malady born of various causes originating with them, for which they have no excuse.

Similar is His statement in Surat Baraʾa: “As for those in whose hearts is a sickness, it only added impurity to their impurity” (9:125).

And likewise His saying