there is no harm in performing wudu from it. This is because the original state is purity, and it is not removed by doubt. If one wishes to know the reality of this, he should throw naphtha into the impure well; if he finds its scent in the water, he knows it has reached it, otherwise not.

And if the water changes in a manner that could potentially be from the impurity, and no other cause for it is known, then it is impure; because adjacency is a cause, so the ruling is attributed to it, whereas anything else remains a matter of doubt.

If one finds water that has changed in a case other than this, and the cause of its change is unknown, it is pure, even if he strongly suspects it is impure; because the original state is purity, and it is not removed by doubt.

If an impurity falls into it and one finds it changed in a manner that could potentially be from that [impurity], it is impure; because the cause of change due to the impurity has been established, so it is not attributed to anything else. If the change could not possibly be from the impurity that fell into it—due to its abundance or scarcity, or because it contradicts its color or taste—then it is pure; because we have not identified a cause for the impurity, so it is analogous to a case where nothing had fallen into it.

Section: If one performs wudu from a small amount of water and prays, then later finds an impurity in it; or performs wudu from a large amount of water, then finds it changed by an impurity, and is in doubt as to whether it occurred before his wudu or after it, then the original state is the validity of his purification and his prayer. If he knows by some indication that it occurred before his wudu, he must repeat it. If he knows that the impurity was present before his wudu, but does not know if it was less than two qullatayn or if it was two qullatayn and then decreased due to use, he must repeat it, because the original state is the deficiency of the water.

Section: If the water of an impure well is drained, and water subsequently springs into it, or is poured into it, it is pure; because the soil of the well is part of the earth, which is purified by the abundance of water flowing over it. If the sides of the well have become impure, is it mandatory to wash them? There are two narrations:

One of them: It is mandatory, because it is a location that an impurity has touched, so it is analogous to the mouth of the well.