the means of restoring its soundness and healing it.

Allah Most High says:

“so that He may make what Satan casts a trial for those in whose hearts is disease”

This is because it gives rise to doubt in them. As for those whose hearts are hardened, their hardness is due to their dryness. The hearts of the former are weakened by illness, so what Satan casts becomes a trial for them; the hearts of the latter are hardened against faith, so it becomes a trial for them as well.

He also says:

“If the hypocrites, those in whose hearts is disease, and those who spread alarming rumours in Medina do not desist”

And likewise:

“and so that those in whose hearts is disease may say”

Their hearts have not died as the hearts of the disbelievers and hypocrites have, nor are they healthy and sound like the sound hearts of the believers. Rather, they suffer from the illness of doubt and desires. Likewise, He says:

“lest he in whose heart is disease should be moved to desire”

This is the illness of lust. Someone with a sound heart would pay no attention to a woman even if she made advances to him. By contrast, a heart diseased with lust, because of its weakness, inclines towards such temptations as present themselves, to a degree corresponding to the severity of its illness. Thus, when women soften their speech, the man whose heart is diseased is moved to desire.

The Quran is a healing for what is in people's hearts. For those whose hearts suffer from the illnesses of doubts and desires, it contains clear proofs that distinguish truth from falsehood. It thereby dispels the illnesses of doubt that corrupt knowledge, understanding, and perception, enabling a person to see things as they truly are. It also contains wisdom and good admonition—through encouragement, warning, and instructive narratives—that restore the heart to soundness. The heart then desires what benefits it and turns away from what harms it. It thus comes to love right guidance and hate error, after having desired error and hated right guidance.