Nu‘man bin Bashir reports: I heard Allah’s Messenger (peace and blessings of Allah be with him) as having said this (and Nu‘man pointed towards his ears with the fingers): What is lawful is evident and what is unlawful is evident, and in between them are the things doubtful which many people do not know. So he who guards against doubtful things keeps his religion and honour blameless, and he who indulges in doubtful things indulges in fact in unlawful things, just as a shepherd who pastures his animals round a preserve will soon pasture them in it. Beware, every king has a preserve, and the things God has declared unlawful are His preserves. Beware, in the body there is a piece of flesh; if it is sound, the whole body is sound, and if it is corrupt, the whole body is corrupt, and hearken it is the heart.
God has clearly defined the things which are lawful and those which are unlawful. Every person has a clear knowledge of them, but in between these lawful and unlawful things, there are certain things which may well be defined as doubtful. The man who shuns the use of such things guards himself against pitfalls, since the use of doubtful things paves the way for falling into the alluring net of evil. At the outset no one dares commit glaring evil; he starts from those acts which apparently have no evil around them, but the intention behind them is no good. He then proceeds slowly and steadily on this dangerous path without realising the enormity of danger lying ahead and ultimately falls victim to evil. The people who do not avoid doubtful things are imperceptibly but irresistibly driven to evil deeds.
The Holy Messenger (peace and blessings of Allah be with him) has explained this process of moral deterioration in a very subtle manner with the help of a simile of the preserve of the king. Like the preserve of the king, evil has a fascination and charm of its own, and the person who hovers about the borderline of evil may at any moment fall into its trap.