What is the ruling when one is certain of both purity and impurity at a prior time, without knowing which state followed the other?

Primary text

When one is certain of having been pure at one point and impure at another point during the time of Dhuhr (noon prayer), without knowing which state occurred subsequent to the other, one must revert to the state prior to the time in question (before Dhuhr). If one was certainly impure before Dhuhr, one is now considered pure. This is because it is certain that a transition from that impurity to purity occurred, but the certainty of the subsequent impurity is not established, as the impurity confirmed after Dhuhr might have preceded the purification. Certainty of purity is not nullified by mere doubt regarding the subsequent impurity. This principle is illustrated by analogy: if two witnesses testify that a man paid his debt of one hundred, and the debtor then presents proof of the creditor admitting he was owed one hundred, the admission does not establish a right for the debtor, as it is possible the admission occurred before the repayment.