in purely physical terms, a person would know where his hand had spent the night. But the sunnah explains one part of another.

Or if it is the final washing by which the impurity is removed, then it is pure. …………

His statement, “Or if it is the final washing by which the impurity is removed, then it is pure”: the pronoun refers to a small quantity of water. It is well known among the jurists that, for an object rendered impure to become pure, it must be washed seven times (1). Thus, all the water that separates from the first through the sixth washings is impure, because it has separated from an impure object.

For example, if a man washes impurity from his garment, the water that runs off from the first through the sixth washings is impure, because it has separated from an impure object while being a small quantity of water. It has therefore come into contact with impurity while being a small quantity, and whatever comes into contact with impurity while being a small quantity becomes impure merely through that contact.

As for the water that separates during the seventh washing, it is pure but not purifying, because it is the final washing by which the impurity is removed. It is pure because it has produced an effect—namely, purification. Once it has purified the object, it becomes like water used to remove ritual impurity. It is not impure, because it has separated from a pure object. As for the water that separates after the eighth washing, it is pure and purifying, because it has produced no effect and has not come into contact with impurity.

This applies when the physical substance of the impurity has disappeared. If it is assumed that the impurity has not disappeared after seven washings, then whatever separates before the physical substance of the impurity has disappeared is impure, because it came into contact with impurity while being a small quantity of water.

His statement, “then it is pure,” is the response to his statement, “And if its taste changes …,” and so on.