How did Ibn al-Qayyim, Ibn Taymiyyah, and Ibn al-Jawzi perceive the diseases of the heart with such detail and precision, and what were their sources besides the Qur’an and Sunnah?

Hearts are of different types: sound (salīm), diseased (marīd), and veiled (aghlaf).

Ahmad narrated, with a weak chain of transmission: "Hearts are four: a barren heart, a veiled heart, an inverted heart, and a heart with two faces (or open)."

Scholars have elucidated the diseases of the hearts, their symptoms, causes, and methods of treatment, relying on the Qur'an, the Sunnah, the traditions of the early Muslims (Salaf), and insight into the human soul.

An example of this is the treatment of envy (hasad), which involves beneficial knowledge. This means realizing that envy harms the envious person in their religion and worldly life, and does not harm the envied person; rather, the envied person benefits from it, and the blessing upon them will not be removed by your envy.

As for beneficial action, its treatment is to force oneself to do the opposite of what envy commands. If it incites you to bear grudges and disparage the envied person, force yourself to praise and commend them. If it leads to arrogance, oblige yourself to be humble towards them. If it prompts you to withhold favors from them, compel yourself to increase your benevolence.

A final piece of advice is to benefit from books such as: "Al-Dā' wa al-Dawā'" and "Ighāthat al-Lahfān" by Ibn al-Qayyim, "Ṣayd al-Khāṭir" by Ibn al-Jawzī, and "Al-Akhlāq wa al-Siyar" by Ibn Hazm.