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.