What does not cleanse effectively may fail to do so because of its smoothness—for instance, if it is exceedingly smooth—or because of its moisture, such as a wet stone or a moist clod of earth; or because the area itself has dried. A stone may be suitable for cleansing effectively, while the area is not suitable for being cleansed effectively.

Other than a bone or dung, …

His statement, “other than a bone or dung,” is a negative condition, and it is the fourth condition, since the word “other than” indicates negation.

The evidence for this is that the Prophet, peace and blessings be upon him, forbade cleansing oneself with a bone or dung, as related in the hadiths of Ibn Mas‘ud (1), Abu Hurayrah (221), Salman (221), Ruwāyfi‘ (2), and others, may Allah be pleased with them.

The rationale is that if the bone is from an animal slaughtered lawfully, the Prophet, peace and blessings be upon him, explained that it serves as food for the jinn. He said to them: “Every bone over which Allah’s name was mentioned will come into your hands with more meat on it than it ever had.” (3) It is not permissible to defile it for the jinn. If it is the bone of a carrion animal, then it is impure and therefore cannot purify.

As for dung, the same evidence cited for bones applies to it. Its rationale is that if it is pure, it is fodder for the beasts of the jinn; and if it is impure, it is not fit to serve as a means of purification.