The hearts are three types:
1. The correct heart that is secure from all desires that oppose the command of Allaah and His prohibitions, and it is secure from all doubts that contradict what He informs. Just as it is secure from worshiping anything else besides Allaah and from seeking judgement 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 worshiping Him.
3. The heart that has some life but also has a defect. So it contains love of Allaah, 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.
So the first type of heart is the living, humble, soft and gentle heart. The second is the dry, harsh and dead heart. The third is the heart which is diseased, it can either be made secure or have its destruction sealed.
All of the diseases of the heart are founded upon desires and doubts. The life of the heart and its illumination is the cause of all good to be found in it and its death and darkness is the cause of all evil to be found in it.
The heart can never be alive and correct except through cognisance of the truth, loving it and preferring it to everything else. There can never be any happiness, joy or correction for it, unless it makes its sole object to worship and desire Allaah alone.
This can never be perfected except through purification of the heart, repentance, and its relinquishing itself from all types of false love and despicable manners. This can never be attained except through striving hard against one's soul that incites towards evil, and bringing it to account and combating the satans from among the jinn by holding fast to Allaah: knowing their plots and objectives, and safeguarding oneself from them through remembrance of Allaah, the Exalted, and seeking refuge with Him from them.
The heart becoming defective and weak revolves around two basic matters: the corruption of knowledge and the corruption of intent. These in turn lead to two destructive illnesses - anger and misguidance. Misguidance being the end result of the corruption of intent. So these two diseases are the lords of all the ailments that afflict the heart. Its cure lies in guidance based on knowledge. Guidance based on knowledge is to know the truth and to follow it. The whole Qur'an is a cure for the these two diseases and others and it contains perfect guidance.
from Diseases of The Hearts & Their Cures by Shaykhul Islaam Ibn Taymiyyah (rahimah-hullaah ta'ala)