The temptations to which the heart is exposed are what cause its sickness. These are the temptations of desires and fancies. The former cause intentions and the will to be corrupted, and the latter cause knowledge and belief to falter.
Hudhayfa ibn al-Yamani, may Allah be pleased with him, said:
The Messenger of Allah (SalAllahu Alaihi Wa Aalihi Wassallam) said, “Temptations are presented to the heart, one by one. Any heart that accepts them will be left with a black stain, but any heart that rejects them will be left with a mark of purity, so that hearts are of two types: a dark heart that has turned away and become like an overturned vessel, and a pure heart that will never be harmed by temptation for as long as the earth and the heavens exist. The dark heart only recognises good and denounces evil when this suits its desires and whims. Sahih Muslim, Kitab al-Iman, 2/170 (with different wording).
He, may Allah bless him and grant him peace, placed hearts, when exposed to temptation, into two categories:
First, a heart which, when it is exposed to temptation, absorbs it like a sponge that soaks up water, leaving a black stain in it. It continues to absorb each temptation that is offered to it until it is darkened and corrupted, which is what he meant by “like an overturned vessel”. When this happens, two dangerous sicknesses take hold of it and plunge it into ruin:
The first is that of its confusing good with evil, to such an extent that it does not recognise the former and does not denounce the latter. This sickness may even gain hold of it to such an extent that it believes good to be evil and vice-versa, the Sunnah to be bida’ and vice-versa, the truth to be false and falsity to be the truth.
The second is that of its setting up its desires as its judge, over and above what the Beloved Prophet Salla Allahu ta’ala ‘alayhi wa ‘aalihi wa Sallam taught, so that it is enslaved and led by its whims and fancies.
Second, a pure heart which the light of faith is bright and from which its radiance shines. When temptation is presented to pure hearts such this, they oppose it and reject it, and so their light and illumination only increase.