for an ʿillah (hidden defect) is a subtle, undermining cause that lurks beneath an outward show of soundness. It may insinuate itself into a chain of transmission that, on the surface, meets every criterion of authenticity. The expert detects it when a single narrator is alone in reporting the ḥadīth or when others contradict him, together with contextual clues that alert the versed critic to a mistake—whether through a broken link (irsāl), a mawqūf stopping, the merging of one report with another, or similar issues—until the critic’s predominant judgement is that the ḥadīth is unsound, or he remains unsure and therefore withholds a ruling.

The way to uncover such a defect is to gather all the available chains, study the differences among their transmitters, and weigh their accuracy and mastery. One of the most common defects is irsāl when the narrator who transmits it as mursal is stronger than the one who provides a connected chain. The ʿillah most often lies in the isnād, though it can also appear in the matn. When it occurs in the isnād it may compromise both chain and text—as with irsāl or rendering a marfūʿ report mawqūf—or it may blemish only the chain while the text remains well-known and authentic, such as the narration of Yaʿlā ibn ʿUbayd al-Thawrī from ʿAmr ibn Dīnār regarding the ḥadīth: “The two parties to a sale have the option [to cancel],” for Yaʿlā erred here; the correct narrator is ʿAbd Allāh ibn Dīnār.

At times the term ʿillah is employed more broadly for other causes of weakness, such as a narrator’s fabrication, heedlessness, or poor memory, and similar factors. Al-Tirmidhī even counted abrogation (naskh) as an ʿillah, and some scholars applied the term to non-detrimental discrepancies, such as someone narrating a ḥadīth mursal that a reliable, precise transmitter has related with a connected chain. Thus they said: “Among the ṣaḥīḥ reports are some that are ṣaḥīḥ yet muʿallal,” just as it is said that certain authentic reports are “ṣaḥīḥ yet shādh (anomalous).” And Allah knows best.