Throughout history, monocratic rulers all over the world were offered written advice on rulership. However, this seemingly universal genre, often referred to as Mirrors for Princes, seldom receives a scholarly treatment that recognizes its transcultural nature. As recently as 2013 it was noted that when it comes to Christian and Islamic Mirrors the historiographical tradition “has taken two extremely similar phenomena and rendered them incommensurable”. This incommensurability is especially questionable in light of our growing awareness of the vivid traffic in thoughts and symbols between the societies that were legatees of the Antique cultural heritage. This paper seeks to address this conceptual problem through transcultural comparisons of Christian and Islamic Mirrors. The transcultural method considers all historiographical divisions of cultures and periods as a discursive reality, which it seeks to question through comparisons that transcend these divisions. This paper uses the discursive separation of Mirrors into a Christian and an Islamic tradition as its basis to select four Mirrors that represent different stages of the two traditions, as historiography has described them. It then goes on to challenge this separation by comparing the texts across the historiographically defined limits. The four Mirrors in question are the Ris?la to the crown prince by ?Abd al-?am?d al-K?tib (d. 750), De Institutione Regia by Jonas of Orléans (760-843), (pseudo-)al-Ghaz?l?'s Na???at al-mul?k (c. early 12th century) and the Castigos e documentos para bien vivir (c. 1293), attributed to Sancho IV (r. 1284-95). To obtain a certain degree of comparability I have chosen texts that were produced under comparable socio-historical circumstances, that is in times of dynastic crises. The four Mirrors are compared for their views on justice, the single-most important regal virtue to emerge from both Christian and Islamic Mirrors. The comparisons reveal not only perennial ideas on justice, but also shed light on the ways in which the authors' interpretations of justice reflect their specific historical context. They show multiple understandings of justice, ranging from the ruler's corrective role vis-à-vis his subjects, to visions of a social and/or cosmological equilibrium. Ultimately, the paper suggests that a consistent model of rulership existed among the societies that could draw on the Late Antique heritage and that the variations of its illustration can mostly be explained without recurrence to concepts like culture or religion.