The Maxim of Certainty and Its Applications to Taharah

The second of the five foundational legal maxims treated in Al-Ashbah wan-Naza'ir — 'certainty is not removed by doubt' (al-yaqin la yazul bish-shakk) — finds its most frequent and practically important applications in the chapter on taharah (ritual purification). Ibn Nujaym devotes considerable space to this maxim and its subsidiary rules, illustrating how a single principle organizes dozens of otherwise seemingly disconnected rulings.

The origin of this maxim is prophetic. The Prophet is reported to have said, in a hadith recorded by Muslim: 'If any of you has a doubt during prayer about whether he passed wind, he should not leave the prayer unless he hears a sound or smells something.' This hadith establishes the principle that a state established with certainty continues to be the presumed state until there is certain or near-certain evidence that it has changed. Doubt alone is insufficient to overturn a prior certainty.

In the domain of taharah, this maxim operates in two directions. First: if a person is certain they performed wudu and then has doubt whether they invalidated it, they remain in a state of ritual purity. The certain state (having wudu) is not removed by the mere doubt (whether it was broken). Second, and conversely: if a person is certain they are in a state of ritual impurity and has doubt whether they performed wudu, they remain impure. In both cases, the presumed state (al-asl) is what was established with certainty, and doubt cannot displace it.

Ibn Nujaym develops a subsidiary maxim from this principle: al-asl baqa' ma kana 'ala ma kana (the default is the continuation of what was, as it was). He then enumerates the legal consequences: if a person doubts whether they washed a limb during wudu, they must wash it again — because the established state is impurity, and doubt cannot move them to purity; if a person doubts how many rak'ahs they have prayed, they build on the lesser number (certainty), not the greater (doubt).

Another subsidiary maxim discussed in this context is: al-asl fi al-ashya' al-ibahah (the default for things is permissibility). This is contrasted with the maxim al-asl fi al-abda' al-hurma (the default regarding private parts is prohibition), meaning that while things are generally presumed permissible, intimate relations are specifically presumed forbidden until a valid contract of marriage establishes their permissibility. The two maxims work together: doubt in a commercial transaction favors permissibility; doubt regarding whether one's marriage contract was valid favors the prohibition.

The maxim also resolves questions about the purity of water. If a person is certain a vessel contained pure water and has doubt whether impurity fell into it, the water remains presumed pure. If there is certainty that impurity entered the water, and then doubt about whether the water has become a flowing stream that self-purifies, the water remains impure until there is certainty of purification.

Ibn Nujaym's treatment in this chapter is a model of legal reasoning by principle: starting from a prophetic hadith, extracting a general maxim, deriving subsidiary maxims, and then systematically demonstrating how they resolve dozens of concrete cases. This approach makes Al-Ashbah wan-Naza'ir both a legal reference and a manual of juristic method.