Is all street mud among the forgiven impurities, or only the mud whose impurity is certain?

The mud on the streets is pure unless its impurity is known, because the default state of things is purity. However, jurists have stated that what becomes impure from it—meaning what has mixed with impurity—is excused. So, if you are certain that impurity has mixed with the mud, it is excused, provided the impurity does not overwhelmingly cover it and the actual substance of the impurity does not touch the person. But if the impurity is overwhelming or its actual substance touches the person, it is not excused, and washing becomes obligatory.