While most studies of epistemology and Islamic law do so through works of legal theory (usūl al-fiqh), this paper argues that the use of substantive law (furū‘ al-fiqh) has much to add to our understanding of premodern Islamic conceptions of knowledge, especially as it concerns court procedure and the testimony of witnesses. For a system of law in which evidence is equated with testimony, court procedure developed as a means of managing the uncertainty of knowledge presented by fallible, biased, socially entangled human witnesses. This relationship is especially well illustrated by early Hanafi jurists like Abu Bakr Aḥmad b. ‘Alī al-Jaṣṣāṣ (d. 370 AH/981 CE), among whose works we find both al-Fusūl fi al-usūl, the first extant, mature work of legal theory, as well as several furū‘ commentaries. By examining the discussion of testimony in one such text, al-Jaṣṣāṣ’s commentary of Aḥmad b. ‘Umar al-Khaṣṣāf’s (d. 261 AH/874 CE) Adab al-qāḍī, this paper will illustrate how Hanafi usūl’s “obsession” with substantive law, to use Murtaza Bedir’s characterization, provides a key means of understanding the definition of knowledge and its practical implications for the judicial process and protection of litigant rights in the early classical period.