This is why the hearts have been classified into three types:
1. The healthy and sound heart which contains no impediment preventing it from accepting the truth, loving it, and giving it preference other than its coming to know of it. Therefore its comprehension of the truth is correct and it accepts it with complete submission.
2. The dead, harsh, and dry heart that does not accept the truth nor submit to it.
3. The diseased heart, when its disease predominates, it joins the ranks of the dead and harsh heart; but if its soundness predominates, it joins the ranks of the truthful and sound heart.
Source: Ighathatu’l-Lahfan fi Masayid al-Shaytan, vol. 1, of Imam ibn al-Qayyim, may Allah have mercy upon him.