It is also said:

“Things become clear through their opposites.”

ʿUmar ibn al-Khattāb, may Allah be pleased with him, used to say:

“I am no deceiver, nor can a deceiver fool me.”

The praiseworthy, sound heart is one that desires good, not evil. Its perfection lies in knowing both good and evil. As for someone who does not recognize evil, this is a deficiency in him, not something deserving praise.

This does not mean that everyone who has tasted disbelief and sins necessarily knows them better and hates them more than someone who has never tasted them. This is not invariably the case: a physician may know more about diseases than the patients themselves. The Prophets—may blessings and peace be upon them—are physicians of faith. They know better than anyone else what sets hearts right and what corrupts them, even though a Prophet may not have tasted the evil that others have tasted.

Rather, the point is that some people, through tasting evil, gain a knowledge of it, an aversion to it, and a love of good when they taste it, which some others do not attain. An example is someone who was a polytheist, a Jew, or a Christian, and came to know the specious arguments, corrupt doctrines, darkness, and evil found in disbelief. Then Allah opened his heart to Islam and acquainted him with its virtues. Such a person may have a greater desire for Islam and a stronger hatred of disbelief than some who have not understood the reality of disbelief and Islam, but instead turn away from aspects of both, or merely follow others in praising the one and condemning the other.