Imām Ibn al-Qayyim (رحمه الله) was asked about the sicknesses of the heart and its cure. Sickness here implies spiritual sickness rather than physical sickness. The heart may fall physically ill. It may be remedied by doctors through medical interventions; however, this danger pertains only to one’s physical life.The second more severe form of illness is spiritual, afflicting the heart due to acts of disobedience. As Allāh (سبحانه وتعالى) says:

فِى قُلُوبِهِم مَّرَضٌ فَزَادَهُمُ ٱللَّهُ مَرَضًا

“In their hearts is a disease (of doubt, disbelief and hypocrisy), so Allāh increased their disease”
(Al-Baqarah, 2:10)

فَلَا تَخْضَعْنَ بِٱلْقَوْلِ فَيَطْمَعَ ٱلَّذِى فِى قَلْبِهِ مَرَضٌ

“Then do not be soft in speech, lest he in whose heart is a disease should be moved with desire” meaning the disease of desire.
(Al-Aḥzāb, 33:32)

The two sicknesses of the heart are: desires and doubts. The sickness of desires is by way of acts of disobedience, while the sickness of doubts is by way of scepticism and uncertainty, the most dangerous of which is doubts within one’s belief, which is only remedied by way of repentance to Allāh (سبحانه وتعالى), righteous actions, and beneficial knowledge of the Book of Allāh and the Sunnah of his messenger (صلى الله عليه وسلم). No medical intervention will remedy them (i.e. doubts and desires). This is the (only) cure for sicknesses (of the heart). Verily Allāh (جلَّ وعلا) sent the Qur’ān as a cure for the hearts and bodies — a physical and spiritual cure. Allāh (عز وجل) says:

قُلْ هُوَ لِلَّذِينَ ءَامَنُوا۟ هُدًى وَشِفَآءٌ

“Say, “It is, for those who believe, a guide and a healing
(Fuṣilat, 41:44)

وَنُنَزِّلُ مِنَ ٱلْقُرْءَانِ مَا هُوَ شِفَآءٌ وَرَحْمَةٌ لِّلْمُؤْمِنِينَ

“And We send down from the Qur’ān that which is a healing and a mercy to those who believe”
(Al-Isrāʾ, 17:82)

Source: al-Jawāb al-Kāfi, p.8
Translated by: Riyāḍ al-Kanadī, Abū ʿUbaydah al-Ḥabashī