What is the ruling regarding a small amount of impurity that is not discernible by sight?
Chapter on what purification is achieved with regarding water
Al-Mughni
Book of Purification
Primary text
The ruling is that an amount of impurity not discernible by sight necessitates impurity just as a discernible amount does. This is because the evidence establishing impurity does not differentiate between a small amount and a large amount, nor between what is discernible by sight and what is not. Differentiating between them is an arbitrary ruling made without evidence.
Supporting text
An opinion attributed to Al-Shafi'i states that an amount of impurity not discernible by sight is excused (ma'fu 'anhu) due to the hardship (mashaqqah) it entails. Furthermore, it is mentioned that if a fly falls into something thin like a cloth or urine, and then lands on clothing, only the spot on the clothing must be washed, implying that the impurity of the fly is of the type not discernible by sight.