Thus, it is permissible to use contaminated oil for lamps, but the author stipulates that this must not be done in a mosque. If it is done in a mosque, then using it for lamps is not permissible.

For example, a man has a vessel of oil into which some impurity falls, rendering it contaminated. He may use it for a lamp in his home, but not in a mosque, because this would result in the mosque being contaminated by the smoke, and contaminating a mosque is forbidden.

This is based on the view that impurity is not purified through transformation. According to the view that impurity is purified through transformation, however, it would be permissible. The scholars have two views on this matter, as mentioned earlier in the chapter on impurities. Transformation is the conversion of one substance into another. An example is a dog that falls into a saltworks and becomes salt—this is possible, for if iron is placed in a saltworks, it becomes salt. Does it remain impure, or not?

The answer depends on the point of disagreement: if we say that transformation does not purify an impure substance, then this mass of salt is impure. But if we say that impurity is purified through transformation, because it has changed from one substance into another, then we say that this mass of salt is pure.

The smoke from an impure substance has undergone transformation from a substance into smoke. Thus, if we hold that an impure substance becomes pure when transformed, we say that it is permissible to use intrinsically impure and contaminated oils for lamps in a mosque and elsewhere.

The author’s words, “It is permissible to use them for lamps”—that is, contaminated oils—imply that it is not permissible to use intrinsically impure oils, such as oil from a carcass, for lamps under any circumstances,