or liquid human excrement mixes with it, it becomes impure even if it has not changed—unless drawing it out would be burdensome. If drawing it out would be burdensome and it has not changed, then it remains purifying. But if drawing it out would not be burdensome, then it becomes impure through the admixture of human urine or liquid human excrement, even if it exceeds two qullahs and has not changed.
Thus, with regard to human urine and liquid human excrement, the determining factor is the difficulty of drawing the water out. If drawing it out would be burdensome and it has not changed, then it remains purifying; but if drawing it out would not be burdensome, it becomes impure merely upon contact. As for the remaining impurities, the determining factor is two qullahs: if it reaches two qullahs and has not changed, then it remains purifying; but if it does not reach two qullahs, it becomes impure merely upon contact.
An example of this is a man who has a waterskin containing water equal to two qullahs, into which donkey droppings fall. Yet the water's taste, colour, and smell do not change; it therefore remains purifying.
Another example: we have a pond containing four qullahs of water. A person urinates a single drop into it, and drawing the water out would not be burdensome; it has not changed, yet it is impure, because the determining factor is the difficulty of drawing the water out.
They cited as evidence that, once water reaches two qullahs, it is not rendered impure except by a change in its properties, the Prophet's saying, “Water is purifying; nothing renders it impure” (1), together with the Prophet's saying: