to purify the mosque of them—whether by keeping the pigeons away, cleaning the mosque, or roofing it over. Prayer in the best, foremost, and greatest of all mosques would also have been invalid because its ground was impure. All these consequences are known with certainty to be untenable.

One of two positions must therefore be accepted: either the droppings are pure without qualification, or their presence is excused, as in the preceding argument. We have already explained why the position that they are pure without qualification is stronger.

The thirteenth argument, which is in fact the sixteenth:

The method of comparison and reasoned explanation. We say—and Allah is the Guide:

Know that the distinction between animals lawful to eat and those unlawful to eat rests on the difference in their essential natures. Allah has called the former wholesome and the latter foul.

The grounds for prohibition include the predatory disposition inherent in an animal: eating such animals causes our bodies to develop from their substance, so that people’s dispositions become those of beasts of prey—or there may be reasons known best to Allah. Alternatively, an animal may be prohibited because of its foul diet, as with birds that eat carrion, or because it is itself repugnant, as with vermin.

We have seen that a wholesome diet affects permissibility, while a foul diet affects prohibition, as established in the Sunnah concerning the meat, milk, and eggs of filth-eating animals: something wholesome becomes forbidden because it feeds on what is foul. The same applies to plants irrigated with impure water or fertilized with dung, according to those who hold this view. We have also seen that not eating food affects the purity of urine or lessens its impurity, as in the case of a young boy who has not yet eaten