and other pure substances that mix with it, so that impurity is not apparent in it,

the response is:

If impurity may or may not be apparent in it, the presumption is that it is not apparent. If we know that pure substances mix with the water and observe a change in it, we attribute that change to those pure substances. A newly arising effect is attributed to a known cause, not to a hypothetical, conjectured one. Indeed, this is established by an explicit text even in matters where the presumption is prohibition, such as game that is wounded and then disappears from sight. An explicit text establishes that it is permissible to eat, even though something else may have happened to it and caused its death. Its death is attributed to the known cause: the wound inflicted by the hunter or his dog. Although there are several opinions on this issue, this is the correct view, established by an authentic, unequivocal text.

The third principle:

Let us suppose that the water has become impure. It has then become impurity on the ground. When impurity is on the ground, whether urine or anything else, the ground is purified by pouring water over it, provided that no actual impurity remains. This is what the Prophet, may Allah bless him and grant him peace, commanded in the hadith of the Bedouin who urinated in the mosque. He said:

“Do not interrupt him”—that is, do not interrupt his urination. “Then pour a bucketful of water over his urine.”

He also said:

“You have only been sent to make things easy; you have not been sent to make things difficult.”

For this reason, Ahmad’s followers and other scholars state that impurity on the ground, in pools, in built basins, and on similar things that cannot be moved or relocated differs in three respects from impurity on movable things, such as bodies, clothing, and vessels.