The Qur'an is a cure for that which is within the heart, and for the one who has sickness of doubt and desire in his heart, for it contains clear proofs that distinguish the truth from falsehood, and remove sickness of false doubts to leave certain knowledge, correct perception and understanding such that the heart sees things accordance to reality. It contains wisdom, goodly exhortations both encouraging good and deterring from evil, and stories which contain lessons that necessarily lead to the correction of the heart by making the heart desire what is good for it and detest what is harmful to it. Hence, the heart is left desiring that which will guide it, (and) hating that which will deviate it.
The Qur'an removes all sickness that invoke false desires until the heart becomes pure and therefore its desires become pure and it returns to the natural state (fitrah) that it was created in, just as the body returns to the natural state upon being treated. The heart will be nurtured with faith and the Qur'an such that it will become strong - for indeed the purification of the heart is like the growint of the body.
Taken from 'Diseases of the Hearts and their Cures' by Ibn Taymiyyah
