This article investigates the philosophical role of proportion as a structural principle in two seemingly disparate domains of Islamic thought: the Qur’anic inheritance rulings in Sūrat al-Nisāʾ and Abū Naṣr al-Fārābī’s theory of musical interval consonance. Moving beyond superficial numerical coincidences, the study argues that both systems operate under a shared "logic of proportional rationality"—a mode of ordering that prioritises proportionate differentiation over arithmetic equality. Drawing on the Greek-Islamic philosophical tradition (Pythagoras, Plato, Aristotle, and al-Fārābī), the article establishes a tripartite analytical framework: (1) ratio as a mechanism for generating order, not merely measuring quantities; (2) the precedence of proportionality over uniformity, where justice and harmony arise from calibrated differences; and (3) unity in multiplicity, where diverse elements cohere through balanced interrelations. In the inheritance verses, fixed fractional shares (e.g., 1/2, 1/3, 1/6, 2:1 for male to female) constitute a legal network that distributes rights according to familial standing and financial responsibility, embodying distributive justice as proportional equity. In al-Fārābī’s Kitāb al-Mūsīqā al-Kabīr, consonance is mathematically grounded in simple ratios (2:1, 3:2, 4:3), where melodic and harmonic coherence emerges not from identical pitches but from their regulated dissimilarities. The comparative analysis reveals that despite fundamental differences in subject matter, purpose, and source of authority (revelation vs. philosophical inquiry), both systems exemplify a civilisational episteme in which proportion serves as the primary organising principle. This study contributes a methodological model for interdisciplinary research in Islamic sciences, demonstrating that concepts like proportion, moderation, and harmony can unveil the internal unity of diverse intellectual traditions.