It has been said regarding al-Shafi'i: That which the eye cannot perceive of impurity is pardoned due to the hardship associated with it. He explicitly stated in one passage that if a fly lands on thin excrement or urine, and then lands on a garment, that spot must be washed; yet the impurity of a fly is among that which the eye does not perceive. Furthermore, the evidence for impurity makes no distinction between a small or large amount of impurity, nor between what the eye perceives and what it does not, so making such a distinction is an arbitrary assertion without evidence. What they mentioned regarding hardship is incorrect, for we only rule the water as impure when we know the impurity has reached it, and with such knowledge, there is no difference in terms of hardship. Moreover, 'hardship' is an underlying wisdom (hikmah) to which it is not permissible to tether a legal ruling, and establishing 'that which the eye cannot perceive' as a standard for it is invalid, for such a matter is only known through explicit scriptural instruction (tawqif) or the Lawgiver's consideration of it in a specific instance, neither of which exists.

Section: As for two pools of water, if one is connected to the other by a conduit containing either a small or large amount of water, they are considered a single body of water, and their ruling is that of a single pool. If both together reach two qullahs, neither is rendered impure except by alteration. If they do not reach that amount, each one becomes impure if impurity falls into either of them, because it is stagnant water connected to one another, resembling a single pool.

Section on Flowing Water: It has been transmitted from Ahmad, may Allah have mercy on him, that which indicates a distinction between flowing water and stagnant water; for he said regarding a bathhouse basin: 'It has been said that it is in the same category as flowing water.' And he said regarding a well that has a source (maddah): 'It is stationary and does not flow; it is not in the same category as that which flows.' Accordingly, flowing water is not rendered impure except by its alteration, because the original state is purity, and we know of no scriptural text (nass) or consensus (ijma') regarding its impurity, so it remains upon the original state of purity. Additionally, it falls under the generality of his (peace and blessings of Allah be upon him) statement: 'Water is purifying; nothing renders it impure,' and his statement: 'Water is purifying; nothing renders it impure, except that which overcomes its smell, taste, or color.'