Al-Istibsar

Al-Istibsar has a narrower job than any of the other Four Books. Rather than gathering the full range of legal hadith the way Kafi, Man La Yahduruhu al-Faqih and Tahdhib al-Ahkam each do, Shaykh al-Tusi built it specifically around cases where two or more transmitted hadith appeared to give conflicting rulings on the same question, presenting the competing narrations side by side and then working through the reasoning a jurist could use to reconcile or rank them. His methods varied by case: sometimes one report was read as abrogating an earlier one, sometimes the apparent conflict was resolved by reading the hadith as addressing different circumstances rather than truly disagreeing, and sometimes the resolution rested on judging one chain of narrators more reliable than the other.

Because al-Tusi drew the great majority of Istibsar's source material from the same pool of hadith he had already treated at greater length in Tahdhib al-Ahkam, the two books function together, Tahdhib as the fuller commentary and Istibsar as a focused companion aimed at the specific problem of contradiction, rather than as two independent collections covering separate ground. Modern editions run to around three volumes, a fraction of Tahdhib's length, reflecting its deliberately narrower brief of isolating and resolving contradiction rather than covering the full span of Islamic law from first principles.

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.