As for pure water becoming indistinguishable from impure water, the disagreement arose only because purification with purifying water is obligatory, whereas purification with impure water is forbidden. Thus, what is obligatory has become indistinguishable from what is forbidden.

Those who disallowed exercising judgment in choosing between them said:

Using impure water is forbidden. Using purifying water, however, is obligatory only when one has the requisite knowledge and ability, which are absent here.

For this reason, they disagreed over whether the purifying water must first be disposed of by mixing it with the other water or pouring it away. There are two well-known views; the sounder is that this is not obligatory, because lack of knowledge is like inability. Al-Shāfiʿī—may Allah have mercy on him—permitted exercising judgment only when both waters were originally pure. In that case, a person would be using water that was originally pure, while doubting whether it had become impure. Its previous state therefore remains presumed to continue.

Those who disagreed with him said:

Water that has become impure through a change in its properties is equivalent to something intrinsically impure. The presumption of its previous state no longer applies once impurity is certain. This is like the case in which one of a man’s two wives becomes forbidden to him through a milk relationship, divorce, or some other cause: she is then equivalent to a woman who was forbidden to him from the outset. The question of the lawful becoming indistinguishable from the unlawful has numerous subsidiary cases.

Now, when pure water cannot be distinguished from impure water, whether we hold that a person should exercise judgment in choosing between them or that he should not, if some water from either one falls on a person’s body, clothing, or food, it does not render it impure. This is because purity is the presumed state, while the impurity of what has come into contact with it is uncertain. We are prohibited from using either water because doing so would amount to preferring one without grounds for doing so. But the impurity of whatever that water touches cannot be established by doubt.

If, however, the two waters come into contact with two garments, one water with each, then one of the garments is ruled impure. If they come into contact with two people’s bodies, would one of those bodies be ruled impure?