HADITH TERMINOLOGY
حسن لغيره
ḥasan li-ghayrihi
A hadith too weak on its own, but reported through several separate weak chains — and their independent agreement lifts it to an acceptable level.
The reasoning is convergence: independent flawed routes reporting the same thing make accidental error or coincidence unlikely. It is called "hasan THROUGH SOMETHING ELSE" precisely because it is not hasan in its own right.
The most important thing about it is its limit. Al-Nawawī states that multiplying chains does NOT automatically rescue a hadith: "if a hadith is narrated through weak routes, it does not follow that hasan results from their sum."
What decides it is the KIND of weakness. Weakness from the poor memory of a truthful, trustworthy narrator is removed by another route, and it becomes hasan. Weakness from irsāl, likewise. "But weakness due to the narrator's moral corruption — corroboration by others has no effect on it at all."
It is disputed whether hasan li-ghayrihi can establish legal rulings or applies only to virtues of deeds — Ibn al-Qaṭṭān al-Fāsī is reported as restricting it to virtues and suspending its use in rulings without further corroboration. How many chains are required is also unsettled: the classical works name no number. The operative rule is qualitative, as above, not numerical — so a claim such as "two weak chains make it hasan" has no basis in the classical sources.