فِي قُلُوبِهِم مَّرَضٌ فَزَادَهُمُ اللَّهُ مَرَضًا ۖ وَلَهُمْ عَذَابٌ أَلِيمٌ بِمَا كَانُوا يَكْذِبُونَ

“In their hearts is a disease (of doubt and hypocrisy) and Allah has increased their disease. A painful torment is theirs because they used to tell lies.”
(Al-Baqarah, 2:10)¹

أَفَلَمْ يَسِيرُوا فِي الْأَرْضِ فَتَكُونَ لَهُمْ قُلُوبٌ يَعْقِلُونَ بِهَا أَوْ آذَانٌ يَسْمَعُونَ بِهَا ۖ فَإِنَّهَا لَا تَعْمَى الْأَبْصَارُ وَلَٰكِن تَعْمَى الْقُلُوبُ الَّتِي فِي الصُّدُورِ

“Have they not travelled through the land, and have they hearts wherewith to understand and ears wherewith to hear? Verily, it is not the eyes that grow blind, but it is the hearts which are in the breasts that grow blind.”
(Al-Ḥajj, 22:46)

ثُمَّ قَسَتْ قُلُوبُكُم مِّن بَعْدِ ذَٰلِكَ فَهِيَ كَالْحِجَارَةِ أَوْ أَشَدُّ قَسْوَةً ۚ وَإِنَّ مِنَ الْحِجَارَةِ لَمَا يَتَفَجَّرُ مِنْهُ الْأَنْهَارُ ۚ وَإِنَّ مِنْهَا لَمَا يَشَّقَّقُ فَيَخْرُجُ مِنْهُ الْمَاءُ ۚ وَإِنَّ مِنْهَا لَمَا يَهْبِطُ مِنْ خَشْيَةِ اللَّهِ ۗ وَمَا اللَّهُ بِغَافِلٍ عَمَّا تَعْمَلُونَ

“Then, after that, your hearts were hardened and became as stones or even worse in hardness. And indeed, there are stones out of which rivers gush forth, and indeed, there are of them (stones) which split asunder so that water flows from them, and indeed, there are of them (stones) which fall down for fear of Allā. And Allāh is not unaware of what you do.”
(Al-Baqarah, 2:74)

فَلَوْلَا إِذْ جَاءَهُم بَأْسُنَا تَضَرَّعُوا وَلَٰكِن قَسَتْ قُلُوبُهُمْ وَزَيَّنَ لَهُمُ الشَّيْطَانُ مَا كَانُوا يَعْمَلُونَ

“When Our torment reached them, why then did they not believe with humility? But their hearts became hardened, and Shayṭān (Satan) made fair-seeming to them that which they used to do.”
(Al-Anʿām, 6:43)

أَلَمْ يَأْنِ لِلَّذِينَ آمَنُوا أَن تَخْشَعَ قُلُوبُهُمْ لِذِكْرِ اللَّهِ وَمَا نَزَلَ مِنَ الْحَقِّ وَلَا يَكُونُوا كَالَّذِينَ أُوتُوا الْكِتَابَ مِن قَبْلُ فَطَالَ عَلَيْهِمُ الْأَمَدُ فَقَسَتْ قُلُوبُهُمْ ۖ وَكَثِيرٌ مِّنْهُمْ فَاسِقُونَ

“Has not the time come for the hearts of those who believe (in the Oneness of Allāh – Islamic Monotheism) to be affected by Allāh’s Reminder (this Qurʾān), and that which has been revealed of the truth, lest they become as those who received the Scripture [the Tawrāh (Torah) and the Injīl (Gospel)] before (i.e. Jews and Christians), and the term was prolonged for them and so their hearts were hardened? And many of them were Fāsiqūn (rebellious, disobedient to Allāh).”
(Al-Ḥadīd, 57:16)

كَلَّا ۖ بَلْ ۜ رَانَ عَلَىٰ قُلُوبِهِم مَّا كَانُوا يَكْسِبُونَ

“Nay! But on their hearts is the rān (covering of sins and evil deeds) which they used to earn.”
(Al-Muṭaffifīn, 83:14)

وَجَعَلْنَا عَلَىٰ قُلُوبِهِمْ أَكِنَّةً أَن يَفْقَهُوهُ وَفِي آذَانِهِمْ وَقْرًا ۚ وَإِذَا ذَكَرْتَ رَبَّكَ فِي الْقُرْآنِ وَحْدَهُ وَلَّوْا عَلَىٰ أَدْبَارِهِمْ نُفُورًا

“And We have put coverings over their hearts lest, they should understand it (the Qurʾān), and in their ears deafness. And when you make mention of your Lord Alone [Lā ilāha ill-Allāh (none has the right to be worshipped but Allāh) Islamic Monotheism] in the Qurʾān, they turn on their backs, fleeing in extreme dislikeness.”
(Al-Isrāʾ, 17:46)

وَقَالُوا قُلُوبُنَا فِي أَكِنَّةٍ مِّمَّا تَدْعُونَا إِلَيْهِ وَفِي آذَانِنَا وَقْرٌ وَمِن بَيْنِنَا وَبَيْنِكَ حِجَابٌ فَاعْمَلْ إِنَّنَا عَامِلُونَ

“And they say: “Our hearts are under coverings (screened) from that to which you invite us, and in our ears is deafness, and between us and you is a screen, so work you (on your way); verily, we are working (on our way).””
(Fuṣṣilat, 41:5)

إِنَّمَا يَسْتَجِيبُ الَّذِينَ يَسْمَعُونَ ۘ وَالْمَوْتَىٰ يَبْعَثُهُمُ اللَّهُ ثُمَّ إِلَيْهِ يُرْجَعُونَ

“It is only those who listen (to the Message of Prophet Muḥammad (صلى الله عليه وسلم)), will respond (benefit from it), but as for the dead (disbelievers), Allah will raise them up, then to Him they will be returned (for their recompense).
(Al-Anʿām, 6:36)

فَإِنَّكَ لَا تُسْمِعُ الْمَوْتَىٰ وَلَا تُسْمِعُ الصُّمَّ الدُّعَاءَ إِذَا وَلَّوْا مُدْبِرِينَ

“So verily, you (O Muḥammad (صلى الله عليه وسلم)) cannot make the dead to hear (i.e. the disbelievers, etc.), nor can you make the deaf to hear the call, when they show their backs, turning away.”
(Al-Rūm, 30:52)

Imām ʿAbd al-Raḥmān ibn Nāṣir al-Saʿdī [d. 1376 AH] comments:

Allāh has mentioned in His book several verses that attribute to the diseases of the heart, its blindness and hardness. Other verses that attribute to it rān: a covering, a screen, confusion (i.e. its envelopment from sins and evil deeds). So know that a heart may exist in a state of either healthiness or sickness. Sickness and other preventers may accumulate within it such that vitality and health is prevented from reaching it. It may also be described as being in a state of pliability and tenderness or hardness.

The Upright Sound Heart

As for the upright sound heart, it is one that is devoid of all forms of sickness. In a healthy state, it demonstrates the acumen of both knowledge and the strength of physical execution and resolve. It belongs to those who know the truth and do not hesitate to act in accordance with it. They also recognise falsehood and, without pause or debate, completely avoid it. This is the truly healthy, upright, clean heart. Such a heart belongs only to people of understanding, intellect, reason, and insight. Those who submit to Allāh with humility, continually seeking repentance from Him.

The Diseased Heart and Its Sicknesses

As for the diseased heart, it occurs when corruption affects one of its two strengths—either its strength of knowledge or strength of action and resolve, or both.

As for the sickness of false desires, it occurs when the heart inclines towards acts of disobedience which represent a form of corruption in the strength of action and resolve in the heart. For, indeed, a truly healthy and upright heart feels no inclination except towards goodness or that which Allāh has made permissible for it. Thus, whenever a heart demonstrates a propensity for disobedience, quickly submitting to engagement in it, it is termed a sick heart. Rapidly giving into any form of disturbance or trial it is subjected to should the means to such things present themselves. As the Most High said:

يَا نِسَاءَ النَّبِيِّ لَسْتُنَّ كَأَحَدٍ مِّنَ النِّسَاءِ ۚ إِنِ اتَّقَيْتُنَّ فَلَا تَخْضَعْنَ بِالْقَوْلِ فَيَطْمَعَ الَّذِي فِي قَلْبِهِ مَرَضٌ وَقُلْنَ قَوْلًا مَّعْرُوفًا

“O wives of the Prophet! You are not like any other women. If you keep your duty (to Allāh), then be not soft in speech, lest he in whose heart is a disease (of hypocrisy, or evil desire for adultery, etc.) should be moved with desire, but speak in an honourable manner.”
(Al-Aḥzāb, 33:32)

As for the heart that is described as hard, it is the one that lacks the tenderness and pliability to accept the truth. Even if it is able to recognise it, it lacks the pliability to submit to it in obedience. In response to religious exhortations so potent that they would soften a piece of iron, this heart remains completely unmoved. Either because it is fundamentally hardened, or because it possesses a corrupt creed that this person has adopted and believes in, to the extent that it has become deeply and firmly rooted in his heart such that submitting to the truth that contradicts his existing beliefs represents a great hardship for him. The heart may also be affected by both [hardness and false desires].

As for rān [covering of sin], veils, and coverings that are ascribed to the heart, they all represent the effects of his actions and crimes. For if he turns away from the truth, or opposes and rejects it after it has come to him, closing the door of righteousness which Allāh has opened before him, he is punished by Him for this action. This punishment takes the form of a complete blocking of the paths to guidance that were previously open before him, easily transversed by him. In response to his arrogant rejection of it, a seal and blockage is placed over his heart. His crimes encompass and surround him, his sins accumulating into a covering, closing his heart, placing a covering that obstructs and blocks him from the truth, until he finds his heart completely shut.

These are the meanings of these terms that are frequently mentioned in the Book of Allāh. The one who comprehends the guiding principles in their interpretation mentioned here will find their meaning clarified to him should he encounter them. It also acts to demonstrate the wisdom and justice of Allāh regarding the manner in which He has chosen to punish these hearts such that Allāh has left them to traverse the path they have chosen and are pleased with.