Another example: A man wakes up and finds wetness on himself, but did not see a wet dream. He then doubts whether it is semen or not. Ghusl is not obligatory upon him on account of this doubt.

If, however, he sees traces of semen on himself and doubts whether they are from the earlier night or the most recent one, he attributes them to the most recent night, because that is certain, whereas what preceded it is doubtful.

The evidence for this is the hadith of Abu Hurayrah and Abdullah ibn Zayd (may Allah be pleased with them) concerning a man who feels something in his stomach and is uncertain whether anything has come out of him or not. The Prophet (peace and blessings be upon him) said: “He should not leave until he hears a sound or perceives an odor.” (1) In the hadith of Abu Hurayrah, it reads: “He should not leave” (566)—that is, leave the mosque—“until he hears a sound or perceives an odor” (566), even though an indication of ritual impurity is present: the rumbling and bloating in his stomach.

His statement, “or vice versa,” means that whoever is certain of ritual impurity but doubts whether he has purified himself, then the basic presumption is that he remains in a state of ritual impurity.

The hadiths of Abu Hurayrah and Abdullah ibn Zayd are cited as evidence for this issue by way of inverse analogy.

Inverse analogy is established in the Shariah. The Prophet (peace and blessings be upon him) said: “In the sexual relations of one of you there is charity.” They said, “O Messenger of Allah, does one of us fulfill his desire and receive a reward for it?” He said: “Yes. Do you not see that if he were to fulfill it unlawfully, he would bear a sin?” They said, “Yes.” He then said: “Likewise, if he fulfills it lawfully, he will have a reward.” (2) ......