Prof. Scott Vitkovic, Ph.D.

Aquinas’ Summa contra Gentiles (SCG), in its apologetic discourse aimed at Jews and Muslims of the Latin Christendom, strategically engages with Islamic and Jewish sages of its scholastic era. Since existing scholarship has not sufficiently qualitatively and quantitatively evaluated Aquinas’ most frequently referenced Islamic and Jewish philosophers in SCG, leaving a significant scholarly lacuna, this study addressed the gap by analyzing Aquinas’ direct references to and argumentative engagements with Avicenna, Averroes, Maimonides, and Avicebron, showing their distribution, Aquinas’ argumentative stance of agreements, partial agreements/disagreements, or disagreements, and purpose of these reference-based arguments, categorized as either Aquinas’ aim to refute the referenced proposition or utilize it to support, build upon, and advance his thesis. The methodology employed a dual analytical approach, consisting of a qualitative textual-philosophical analysis of the SCG text in the scholarly authoritative Editio Leonina, cross-verified against Vansteenkiste’s studies and Busa’s Index Thomisticus, followed by a quantitative assay. The results revealed that Aquinas directly referenced the Islamic and Jewish philosophers 61 times in the SCG’s 415 Capitula. Islamic philosophers (95%) Averroes (64%) and Avicenna (31%) are the most prominent. In contrast, Jewish philosophers, Maimonides and Avicebron, comprise only a minor fraction. These references are concentrated in SCG II, followed by SCG III and SCG I. The SCG IV, which deals with the Latin Catholic dogmatic theology, has only one direct reference to Avicenna. Aquinas often engaged with these philosophers through argumentative disagreements (81%), primarily based on Averroes’ arguments and secondarily on Avicenna’s. However, he never refuted the relatively few agreements (14%) and partial agreements (5%), instead utilizing them to build upon, advance, and support his thesis. Aquinas never agreed with Maimonides and Avicebron and always used these argumentative disagreements to refute their philosophical propositions. This pattern exhibits a strong correlation between Aquinas’ argumentative agreement level and purpose of direct reference-based argument, illustrating the Thomistic method of theological discernment, characterized by Aquinas’ selective appropriation, adaptation, and adoption of the other two Abrahamic traditions’ philosophies when compatible with the Latin Church’s teachings, and systematic refutations when incompatible, terming them ‘errors’ that must be countered. This study demonstrates that the Thomistic theological synthesis of Islamic and Jewish philosophy in Aquinas’ SCG Sacra Doctrina not only strengthened Thomistic theology and defended the Roman Catholic faith but also succeeded in bridging the three Abrahamic monotheistic traditions, Christian, Islamic, and Jewish, through their shared Peripatetic philosophy of universal truth, the truth that commences in God.

This thesis undertakes a comparative analysis of the philosophical contributions of Avicenna and Aquinas, focusing on the influence of Avicenna's Sufficientia on Aquinas' De Ente et Essentia, Summa Contra Gentiles, and Summa Theologiae. The central theme of this study is the transmission and adaptation of Avicenna's metaphysics, especially the distinction between essentia (مَاهِيَّة) and esse (وجود) within Aquinas' Sacra Doctrina. This Avicennian distinction, first integrated into De Ente et Essentia, became the basis for Aquinas' demonstration and proof of God's existence in his Summa Contra Gentiles I. and Summa Theologiae I. This thesis also investigates the bearing of Aristotle's Metaphysica on both Avicenna and Aquinas. It distinguishes between Aristotelian and Avicennian influences on Aquinas' metaphysical theology of Sacra Doctrina concerning the subject matter of metaphysics, the definition of God (or lack thereof), the essence and existence of God, the demonstration of God's existence, the proofs of God's existence, the unity and oneness of God, and divine thought and intellect. This thesis employed a rigorous methodology that combined historical textual analysis, philosophical exegesis, and comparative conceptual analysis, examined primary texts in Koine Greek, Arabic, and Latin, alongside authoritative English translations, and traced the transmission of Aristotle's and Avicenna's metaphysics in Aquinas' texts. Additionally, the study engaged extensively with both ancient and contemporary scholarly writings on Aristotle, Avicenna, and Aquinas, contextualized and interpreted their metaphysical arguments, and showed that Avicenna's metaphysics was integral to Aquinas' knowledge of the nature of God as the necesse esse (واجِبُ الوُجُود) and ipsum esse subsistens (أن يكون قائما بنفسه) whose essence is identical to His existence. These concepts informed Aquinas' doctrines on God's divine simplicity and unity and impacted the fundamental doctrines of the Roman Catholic Church's dogmatic and natural theology. This thesis contributed to comparative medieval philosophy by demonstrating the profound philosophical influence of Avicenna's Islamic philosophy on Thomism. The engagement between these two great philosophers reflects the universality of metaphysical inquiry and demonstrates its enduring relevance to Shia Islam and Western Christianity. Avicenna and Aquinas left an indelible mark on the development of medieval scholastic philosophy, proving that Peripatetic metaphysics transcends temporal, geographic, linguistic, cultural, and religious boundaries in the interest of the ultimate Truth.