‌Context & Objective: The relationship between morality, the construction of a legal system, and legal validity remains a foundational question in legal philosophy, highlighted after the Second World War when Nuremberg and Tokyo defendants invoked domestic laws to justify their actions. While Gustav Radbruch argued that extreme injustice invalidates positive law, H.L.A. Hart separated legal validity under rules of recognition from moral obedience, contending in the post-war informer case that retroactive punishment violated fidelity to law. Replying to Hart, Lon Fuller argued that positivism cannot explain the moral obligation of fidelity to law and introduced the inner morality of law, comprising eight procedural principles required for governance through rules. In Islamic legal theory, Shiʿi scholars of Uṣūl al-Fiqh (the principles of Islamic jurisprudence) reject conjectural reasoning like analogy, yet accept ḥosn va qobḥ-e ʿaqlī (rational goodness and badness, the intellect's capacity to discern the praiseworthiness or blameworthiness of voluntary actions) as both a foundation of Sharia and an inferential tool. This study investigates how Fuller's inner morality and selected rational rules in Imami jurisprudence govern the construction, expression, comprehension, and compliance with rulings, and what effects result from violating them.
 ‌Method & Approach: Utilizing a doctrinal method and a comparative approach, this research re-examines primary sources alongside the historical and theoretical contexts of both traditions. To avoid conceptual conflation, the framework distinguishes between the realization of a legal system capable of guiding behavior, the legal validity of a rule under accepted criteria of recognition, and moral justification. Concurrently, in Imami jurisprudence, the study separates the ontological possibility of divine obligation, ḥojjīyat (the probative authority of evidence attributing a ruling to the Divine Lawgiver), and tanjīz (the binding actualization of a duty such that violation warrants punishment). Selected Imami rational rules—covering the commander, addressee, subject of obligation, and discourse interpretation—are compared against Fuller's eight principles across four criteria: subject matter, justificatory basis, scope of application, and the effect of violation.
 ‌Findings: In Imami theology and jurisprudence, Sharia is objective and rational because divine rulings follow maṣāleḥ va mafāsed-e nowʿīyeh (general species-wide benefits and harms inherent in actions). Because legislation and communication are voluntary acts of a Wise Lawgiver, rational rules govern both substantive legislation and textual interpretation. Comparing these rules with Fuller's principles reveals three areas of functional overlap alongside structural differences. First, Fuller's possibility of compliance parallels the Imami rules of emtenāʿ-e taklīf be maḥāl (impossibility of obligating the impossible) and qobḥ-e taklīf-e mā lā yoṭāq (impropriety of obligations beyond human capacity). Second, Fuller's promulgation requirement partially overlaps with qobḥ-e ʿeqāb belā bayān (impropriety of punishment without prior declaration); however, public promulgation differs from individual receipt of proof, as uncommunicated rulings remain valid in reality under Abu al-Qasim al-Khoei's view and only lose punitive bindingness, whereas Muhammad Baqir al-Sadr rejects this rule under ḥaqq al-ṭāʿah (God's inherent right to obedience covering probable duties). Third, Fuller's clarity principle aligns with Imami interpretive rules—including the impropriety of meaningless speech, the duty to provide contextual indicators for non-apparent meanings, and qobḥ-e taʾḫīr-e bayān az vaqt-e ḥājat (impropriety of delaying explanation past the time of need)—though meaningfulness, actual clarity, and the validity of apparent meaning remain distinct. Conversely, Imami rules on universal commands, evidentiary conflicts, nasḫ (abrogation of rulings), and divine wisdom are not direct equivalents to Fuller's generality, non-contradiction, constancy, and official congruence.
 ‌Conclusion: The comparison demonstrates that rational goodness and badness in Imami legal reasoning operate primarily as procedural safeguards rather than direct determinants of substantive behavioral content. Just as Fuller's inner morality establishes procedural prerequisites for governing conduct through rules without dictating the substantive justice of every law, Imami rational principles regulate the conditions for attributing rulings to a Wise Lawgiver, validating textual interpretation, and establishing accountability. Nevertheless, shared functional requirements regarding compliance capacity, communication, and intelligibility do not entail identical foundations or legal consequences. For Fuller, total failure in these principles undermines the existence of a legal system itself, whereas in Imami jurisprudence, violating rational standards negates the attribution of a specific ruling, invalidates an interpretation, or excuses an individual from punishment without negating the underlying divine ruling.