"...Indeed there is in the body a piece of flesh which if it is sound thence the whole body is sound, and if it is corrupt then the whole body is corrupt. Indeed it is the heart."
Sahih al-Bukhari 52, In-book reference : Book 2, Hadith 45
The hearts are of three types:
1. The correct heart that is secure from all desires that oppose the command of Allah and His prohibitions, and it is secure form all doubts that contradict what He informs. Just as it is secure from worshipping anything else besides Allah and from seeking judgment from any person other than His Messenger.
2. The dead heart, this being the opposite of the correct heart containing no life, neither knowing its Lord nor worshipping Him.
3. The heart that has some life but also has a defect. So it contains love of Allah, faith in Him, sincerity and trust towards Him from those things that are essential to it remaining alive. It also contains the love of vain desires and preference for them, despicable morals and manners from those things that cause it to die, and it is continuously wavering between these two conditions.
The Quran removes all the sicknesses 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 Quran such that it will become strong - for indeed the purification of the heart is like the growing of the body.
Concerning the Ailments of the Hearts and their Cures
Indeed all praise is due to Allah, we seek His help, and we seek His forgiveness, and we seek refuge in Allah from the evil of our sows and the evil of our actions. Whomsoever Allah guides, none can misguide, and whomsoever Allah misguides, none can guide. I bear witness that there is no deity worthy of worship except for Allah, the One Who has no partner, and I bear witness that Muhammad is His servant and Messenger.
Allah the Exalted said about the hypocrites,
فِي قُلُوبِهِم مَّرَضٌ فَزَادَهُمُ اللَّهُ مَرَضًا وَلَهُمْ عَذَابٌ أَلِيمٌ بِمَا كَانُوا يَكْذِبُونَ
In their hearts is a disease and Allah has increased their disease. [Surah al-Baqarah (2) : 10]
لِّيَجْعَلَ مَا يُلْقِي الشَّيْطَانُ فِتْنَةً لِّلَّذِينَ فِي قُلُوبِهِم مَّرَضٌ وَالْقَاسِيَةِ قُلُوبُهُمْ وَإِنَّ الظَّالِمِينَ لَفِي شِقَاقٍ بَعِيدٍ
That He may make what is thrown in by Satan a trial for those in whose hearts is a disease and whose hearts are hardened. [Surah al-Hajj (22) : 53]
لَّئِن لَّمْ يَنتَهِ الْمُنَافِقُونَ وَالَّذِينَ فِي قُلُوبِهِم مَّرَضٌ وَالْمُرْجِفُونَ فِي الْمَدِينَةِ لَنُغْرِيَنَّكَ بِهِمْ ثُمَّ لَا يُجَاوِرُونَكَ فِيهَا إِلَّا قَلِيلًا
If the hypocrites, and those in whose hearts is a disease, and those who spread false news among the people of Madinah cease not, We shall certainly let you overpower them, then they will not be able to stay in it as your neighbours but for a little while. [Surah ah-Ahzab (33) : 60]
وَمَا جَعَلْنَا أَصْحَابَ النَّارِ إِلَّا مَلَائِكَةً وَمَا جَعَلْنَا عِدَّتَهُمْ إِلَّا فِتْنَةً لِّلَّذِينَ كَفَرُوا لِيَسْتَيْقِنَ الَّذِينَ أُوتُوا الْكِتَابَ وَيَزْدَادَ الَّذِينَ آمَنُوا إِيمَانًا وَلَا يَرْتَابَ الَّذِينَ أُوتُوا الْكِتَابَ وَالْمُؤْمِنُونَ وَلِيَقُولَ الَّذِينَ فِي قُلُوبِهِم مَّرَضٌ وَالْكَافِرُونَ مَاذَا أَرَادَ اللَّهُ بِهَٰذَا مَثَلًا كَذَٰلِكَ يُضِلُّ اللَّهُ مَن يَشَاءُ وَيَهْدِي مَن يَشَاءُ وَمَا يَعْلَمُ جُنُودَ رَبِّكَ إِلَّا هُوَ وَمَا هِيَ إِلَّا ذِكْرَىٰ لِلْبَشَرِ
...and that no doubts may be left for the People of the Book and the believers, and that those in whose hearts is a disease and the disbelievers may say, 'What does Allah intend by this parable?' [Surah al-Muddaththir (74) : 31]
يَا أَيُّهَا النَّاسُ قَدْ جَاءَتْكُم مَّوْعِظَةٌ مِّن رَّبِّكُمْ وَشِفَاءٌ لِّمَا فِي الصُّدُورِ وَهُدًى وَرَحْمَةٌ لِّلْمُؤْمِنِينَ
...There has come to you good advice from your Lord, and a healing for that which is in the hearts, a guidance and a mercy for the believers. [Surah Yunus (10) : 57]
وَنُنَزِّلُ مِنَ الْقُرْآنِ مَا هُوَ شِفَاءٌ وَرَحْمَةٌ لِّلْمُؤْمِنِينَ وَلَا يَزِيدُ الظَّالِمِينَ إِلَّا خَسَارًا
And We send down from the Quran that which is a healing and mercy to those who believe, and it increases the wrong-doers in nothing but loss. [Surah al-lsra (17) : 82]
قَاتِلُوهُمْ يُعَذِّبْهُمُ اللَّهُ بِأَيْدِيكُمْ وَيُخْزِهِمْ وَيَنصُرْكُمْ عَلَيْهِمْ وَيَشْفِ صُدُورَ قَوْمٍ مُّؤْمِنِينَ
وَيُذْهِبْ غَيْظَ قُلُوبِهِمْ وَيَتُوبُ اللَّهُ عَلَىٰ مَن يَشَاءُ وَاللَّهُ عَلِيمٌ حَكِيمٌ
...and heal the breast of a believing people and removes the anger of their hearts ... [Surah Tawbah (9) : 14-15)]
The Praise of the scholars for the author Ibn Taymiyyah:
Ibn Hajr al-'Asqalani. said,
The Shaikh of our Shaykhs, Abu al-Yu'mari [ibn Sayyid an-Nas] said in his biography of ibn Taymiyyah, `al-Mizzi encouraged me to express my opinion on Shaykh al-Islam Taqi ad-Din. I found him to be from those who had acquired a fortune of knowledge in the sciences that he had. He used to completely memorise and implement the Sunan and Athar (narrations). Should he speak about tafsir, he would carry its flag, and should he pass a legal ruling in fiqh, he knew its limits. Should he speak about a hadith, he was the seat of its knowledge and fully cognisant of its narrations. Should he give a lecture on religions and sects, none was seen who was more comprehensive or meticulous than he. He surpassed his contemporaries in every science, you would not see one like him and his own eye did not see one like himself...'
Ibn al-Qayyim said,
Allah knows, I have never seen anyone who had a better life than his. Despite the difficulties and all that expunges comfort and luxury, nay, things completely opposite to them; despite imprisonment, intimidation and oppression, ibn Taymiyyah had a purer life than anyone could. He was the most generous, the strongest of heart and the most joyful of souls, with the radiance of bliss in his face. When we were seized with fear and our thoughts turned negative, and the earth grew narrow for us, we would go to him. No sooner did we look at him and hear his words, all these feelings would leave us to be replaced by relief, strength, certainty and tranquillity.'
Al-Hafiz al-Mizzi said,
I have not seen the likes of him and his own eye had not seen the likes of himself. I have not seen one who was more knowledgeable than he of the Book and the Sunnah of His Messenger, nor one who followed them more closely.
Ibn Kathir said,
The least he would do when he heard something was to memorise it and then busy himself with learning it. He was intelligent and had much committed to memory, he became an linden in tafsir and everything linked to it and knowledgeable in figh. Indeed it was said that he was more knowledgeable of the fiqh of the madhhabs than the followers of those very same madhhabs in his time and other than his time. He was a scholar in Usid and the branches of the religion, in grammar, the language and other textual and intellectual sciences.... no scholar of a science would speak to him except that he thought that that science was the specialty of ibn Taymiyyah. As for haclith then he was the carrier of its flag...