Is it possible to think philosophically about God? The very idea may seem oxymoronic. But premodern philosophers from Plato to Plethon and probably most philosophers until relatively recently would have seen no contradiction. “Theology” was Aristotle’s term for what we call metaphysics, and reference to a divine being or beings was an indispensable part of philosophy within the later tradition, closely bound up with philosophical ontology – theories of the very nature of “being,” which were treated as a precondition for thinking about anything else that exists or could exist. “Theology” was “first philosophy” (Aristotle, Metaphysics Ε, 1026a19 and 24).

Now, not everyone is a philosopher, and pagans, Jews, Christians, Muslims, and many others developed accounts of the divine that we might reasonably call “theologies” apart from any of the intellectual traditions we usually call philosophy. But even in these cases, we must tread carefully for two reasons. First, authors we think of as theologians were often steeped in Aristotelian, Stoic, Neoplatonic, or other ideas that historians typically refer to as “philosophy.” Such a theologian, for example, was Gregory of Nazianzos, as Susanna Elm has amply demonstrated (Sons of Hellenism, Fathers of the Church, Berkeley, 2012). Secondly, even when a theologian is not obviously working within a philosophical tradition, we should nevertheless take seriously the possibility that an author may be engaged in “philosophy,” not philosophy as tradition but philosophy as practice, a practice that all those traditions share: rigorous reflection that seeks to answer questions about the world using some form of demonstrative reasoning.

Giovanni Mandolino’s rich, rigorous, and thought-provoking new monograph provides an opportunity to reflect on how philosophers’ own prior theological commitments (inherited from broad elite cultural assumptions and specific doctrinal communities alike) might have shaped their philosophical theology more generally. Or, to put it in the more conventional terms favored by Mandolino, to reflect on the relationship between “philosophy” and “theology” within a single author’s body of work. The book revolves around Mandolino’s re-edition, translation, and extensive philosophical and philological commentary on a treatise, written in 940 ce, on the precise sense in which God is “one,” by the Miaphysite Christian philosopher Yaḥyā ibn ʿAdī (893–974). Yaḥyā, like the Muslim philosopher al-Fārābī (d. 950), was a student of the Nestorian Christian Abū Bishr Mattā ibn Yūnus (d. 940). His Treatise on Divine Unity, as Mandolino argues, presents a novel argument for the claim that God is simultaneously characterized by unicity in one sense and multiplicity in another sense – a claim that supports his Christian community’s trinitarian theology, as Mandolino (but not Yaḥyā) repeatedly underscores. Mandolino’s edition, translation, and detailed commentary make this treatise much more widely accessible to non-specialists.

That is only half of what Mandolino’s book does, however; the entire first part of the monograph (pp. 17–165) offers a major contribution to intellectual history in its own right: a detailed biographical study of Yaḥyā (Chapter 1), an analysis of his treatise on the oneness of a triune God (Chapter 2), and an account of how his ideas were in conversation with key notions put forth by the famous Arabophone philosophers of his age, in particular al-Kindī (d. ca. 870) and al-Fārābī, both Muslims (Chapter 3). This first part of the monograph engages respectfully and constructively with the relevant scholarship. Chapters 2 and 3 build on Mandolino’s own erudite commentary on the treatise in Part II, placing Yaḥyā in the context of Arabic Christian apologetics, Greek Aristotelian commentators and Neoplatonists (and Plato and Aristotle themselves), Arabic Muslim mutakallimīn, the Arabophone Aristotelians and Neoplatonists of Baghdad, Yaḥyā’s other works, Greek patristics, and later Arabophone Muslim philosophers, above all Ibn Sīnā (a.k.a. Avicenna, 980–1037).

The step-by-step analysis of Yaḥyā’s treatise in chapter 2 traces how Yaḥyā, after opening with an account of various views on the sense in which God is “one,” goes on to construct and justify what he presents as the only coherent solution via proof by exhaustion. The treatise, steeped in Aristotelian terminology, comes across as a classic Aristotelian exposition in the mold of Aristotle’s own treatises, which typically begin with an account of how Aristotle’s predecessors addressed a given question before offering Aristotle’s own view, resting on a precise understanding of terminology and presented as a sort of moderate compromise solution. Chapter 3, meanwhile, makes clear that Yaḥyā’s treatise, while drawing on authors remembered today as Christian theologians, was above all in dialogue with the work of well-known Arabophone Muslim philosophers like al-Kindī (ninth century) and Yaḥyā’s contemporary al-Fārābī. Deep within chapter 3, the reader is treated to an especially fascinating discussion of how Yaḥyā’s treatise may have shaped subsequent Arabic philosophy, including Ismailis like al-Sijistānī (d. ca. 975) and al-Kirmānī (996–1021), eventually helping Ibn Sīnā to formulate his “famous … distinction between essence and existence” (pp. 126–132). Here we have a philosophical treatise, partially motivated by Christian trinitarian teachings, leading to metaphysical insights within the Aristotelian tradition that have nothing to do with trinitarian theology.

This brings us to the central question that drives Mandolino’s study: the relationship between “philosophy” and “theology.” Mandolino acknowledges that this terminology is problematic (p. 5) but adopts it anyway. Yet his astute analysis of Yaḥyā’s treatise suggests that we might consider abandoning this dichotomy altogether. Instead of treating Yaḥyā’s treatise as a sort of hybrid between Christian (implicitly non-rational or less-rational) “theology” and secular (rational) philosophy, we might learn more by thinking of this and the other philosophical works Mandolino cites as philosophical treatments of trinitarian and unitarian theology. In other words, we might think of “theology” above all as a topic, rather than tacitly back-projecting onto tenth-century Baghdad the disciplinary divide that emerged at Western medieval European universities, most famously at Paris, between the discipline of theology and that of (Aristotelian) philosophy, each with its peculiar methodologies, authoritative texts, and preferred answers to various questions, including ontological – theological – questions. We will no longer feel the need to insist that Yaḥyā and other Christian philosophers (but not their Muslim or pagan colleagues) were “biased” (p. 81) in reaching a conclusion. Of course, Yaḥyā was in a sense “biased,” but the same could surely be said of most philosophers. We will see that Yaḥyā and his Christian colleagues were no more “biased” than other philosophers who reached conclusions that suited their prior views about reality, starting with Aristotle himself, who often seems committed to explaining rigorously how the world must in fact be as it seemed to him to be.

Mandolino’s choice to work within the traditional historiographical framework will no doubt make his book more palatable for some readers. But Mandolino’s meticulous readings and his willingness to devote extensive space to explicitly Christian literature in a book on the history of philosophy invite us to revise or replace the framework instead. It took the efforts of at least a generation of scholars to show that medieval Islam was perfectly compatible with philosophy (by actually reading philosophical texts written by Muslims; see Maria Mavroudi, “The Modern Historiography of Byzantine and Islamic Philosophy: A Comparison,” Al-Masāq 33.3). Mandolino’s masterful treatment of an incisive medieval philosophical essay readies us to do the same for the medieval Christianity of Byzantium and the Islamic world.