Either the area is confined, or it is extensive.

If it is extensive, he should determine, on the basis of what seems most likely, the area that the impurity affected and wash it, because washing the entire extensive area would be difficult.

But if it is confined, he must wash until he is certain that the impurity has been removed.

An example of this is when impurity gets on one of the two sleeves of a garment, but one does not know which sleeve it affected. In that case, both sleeves must be washed, because one cannot be certain that it has been removed except by doing so.

The same applies if you knew which of the two it was, but then forgot: both of them must be washed.

….

The author’s statement indicates that determining the more likely possibility is not permissible, even when it is possible, because certainty and assurance are indispensable.

The correct view, however, is that it is permissible to determine what is most likely, due to the Prophet’s statement, peace and blessings be upon him, regarding uncertainty in prayer: “Let him determine what is correct, then complete the prayer accordingly.” (1)

Accordingly, if there is scope for determining what is most likely, you should determine which of the two sleeves the impurity affected, then wash it.

For example, if you passed by impurity on your right and some of it got on you, but you did not know which sleeve it touched, then it is most likely that it was the right sleeve. You must therefore wash it, rather than the left sleeve.

But if there is no scope for determining what is most likely, then wash both sleeves, because you cannot be certain that the impurity has been removed except by doing so. Thus, there are four cases: