Closing the Four Books, Not the Argument

With al-Istibsar's completion in the eleventh century CE, the canon later tradition would call the Four Books was closed: Kafi, Man La Yahduruhu al-Faqih, Tahdhib al-Ahkam and al-Istibsar, compiled across roughly a century and a half by three different scholars working in Baghdad, Qom and Najaf. No fifth collection was ever added to stand alongside them at the same level of authority, though extensive later hadith compilations, biographical dictionaries evaluating individual narrators, and commentary literature on all four books have continued to be produced without a break into the present. What the Four Books did not close was disagreement over how to use them.

The Akhbari school, strongest from roughly the seventeenth to nineteenth centuries, favored treating the Four Books as reliable in something close to their transmitted form; the Usuli school that came to dominate modern Twelver Shia jurisprudence instead treats every hadith in all four, al-Istibsar included, as raw material to be weighed and reasoned over using the formal tools of usul al-fiqh before a ruling can be derived, a methodological difference that keeps active argument attached to material otherwise regarded as closed.