وَتَطْهُرُ النَّجَاسَةُ الَّتِي يَجِبُ غَسْلُهَا عَلَى وَجْهَيْنِ: فَمَا كَانَ لَهُ عَيْنٌ مَرْئِيَّةٌ فَطَهَارَتُهَا زَوَالُ عَيْنِهَا، إِلَّا أَنْ يَبْقَى مِنْ أَثَرِهَا مَا يَشُقُّ إِزَالَتُهَا.
Translation: Impurity that requires washing is purified in two ways. Where it has a visible substance, purification is by removal of that substance, except an effect that is difficult to remove.
A visible substance is impurity whose material mass can be seen, such as adhering filth on cloth. The main objective is to eliminate the mass.
A stubborn residual colour or trace that cannot be removed without serious difficulty is distinguished from the continued presence of the actual filth.