Third of Four, Third Read

Man La Yahduruhu al-Faqih is conventionally counted third among the Four Books, after Kafi and before Shaykh al-Tusi's Tahdhib al-Ahkam and al-Istibsar, though al-Saduq compiled it independently of al-Tusi's later pair rather than in direct response to them. Its legal content overlaps substantially with Furu al-Kafi, the legal section of Kafi, since both authors drew on much the same pool of transmitted hadith circulating among Twelver scholars in the tenth century, but the two books rarely agree in exact wording or arrangement, giving later jurists two independently compiled witnesses to compare against each other rather than one source repeated twice. Al-Saduq included a preface explaining his selection principles, naming the sources and teachers he trusted, which later commentators have used to assess how the book's comparatively light sourcing should be weighed against Kafi's fuller chains.

In practice the Four Books were never meant to be read alone: a working Twelver jurist deriving a ruling on any given question is expected to consult all four together, along with the biographical and chain-of-transmission literature built up around them over the following centuries, rather than treat any single collection as sufficient by itself.