In the 11^th century, a revival of philosophical thought began as a result of the increasing contact between different parts of the Western world and the general reawakening of cultural interest that culminated in the Renaissance. The works of Plato, Aristotle, and other Greek thinkers were translated by Arab scholars and brought to the attention of philosophers of Western Europe. Muslim, Jewish, and Christian interpreted and clarified these writings in an effort to reconcile philosophy with religious faith and to provide rational grounds for their religious beliefs. Their labors established the foundations of Scholasticism.

    Scholastic thought was less interested in discovering new facts and principles than in demonstrating the truth of existing beliefs. Its method was therefore dialectical, or argumentative. Intense concern with the logic of argument led to important developments in logic as well as theology. The 11^th century Arab physician Avicenna united Neoplatonic and Aristotelian ideas with Muslim religious doctrine, and the Jewish poet Solomon ben Yehuda Ibn Gabirol made similar synthesis of Greek thought and Judaism. The ecclesiastic and scholastic philosopher Anselm of Canterbury adopted Augustine's view of the relation between faith and reason and combined Platonism with Christian theology. Supporting the Platonic theory of Ideas, Anselm argued in favor of the separate existence of the universals, or common properties of things. He thus established the position of logical realism on one of the most vigorously disputed issues of medieval philosophy.

    The contrary view, known as nominalism, was formulated by the Scholastic philosopher Roscelin, who maintained that only individual, concrete objects exist and that the universals, forms and ideas, under which particular things are classified, constitute mere sounds or marks, rather than intangible substances. When he argued that the Trinity must consist of three separate beings, his views were deemed heretical and he was forced to recant in 1902. The French Scholastic theologian Peter Abelard, whose tragic love affair with Heloise in the 12^th century is one of the most memorable romantic stories in medieval history, proposed a compromise between realism and nominalism known as conceptualism, according to which universals exist in particular things as properties and outside of things as concepts in the mind. Abelard maintained that revealed religion must be justified by reason. He developed an ethics based on personal conscience that anticipated Protestant thought.

The Spanish-Arab jurist and physician Averroes, the most noted Muslim philosopher of the Middle Ages, made Aristotelian science and philosophy a powerful influence on medieval thought with his lucid and scholarly commentaries on the works of Aristotle. He earned himself the title "the Commentator" among the many Scholastics who came to regard Aristotle as "the Philosopher." Averroes attempted to overcome the contradictions between Aristotelian philosophy and revealed religion by distinguishing between two separate systems of truth, a scientific body of truths based on reason and a religious body of truths based on revelation. His view that reason takes precedence over religion led to his exile in 1195. Averroes' so-called double-truth doctrine influenced many Muslim, Jewish, and Christian philosophers; it was rejected, however, by many others, and it became an important issue in medieval philosophy.

The Jewish rabbi and physician Moses Maimonides, one of the greatest figures in Judaic thought, followed Averroes in uniting Aristotelian science with religion but rejected the view that both of the two conflicting systems of ideas can be true. In his Guide to the Perplexed (1180), Maimonides attempted to provide a rational explanation of Judaic doctrine and defended religious beliefs (such as the belief in the creation of the world) that conflicted with Aristotelian science only when he was convinced that decisive evidence was lacking on either side.

The English Scholastic theologian Alexander of Hales and the Italian Scholastic philosopher Saint Bonaventure, both philosophers of the 13^th century, combined Platonic and Aristotelian principles and introduced the concept of substantial form, or nonmaterial substance, to account for the immortality of the soul. Bomaventure's view tended toward pantheistic mysticism in making the end of philosophy the ecstatic union with God.

The German Scholastic philosopher Saint Albertus Magnus was the first Christian philosopher to endorse and interpret the entire system of Aristotelian thought. He studied and admired the writings of the Muslim and Jewish Aristotelians and wrote encyclopedic commentaries on Aristotle and the natural science of his day. Albertus Magnus died in 1280. The English monk Roger Bacon, one of the first Scholastics to take an interest in experimental science, realized that a great deal remained to be learned about nature. He criticized the deductive method of his contemporaries and their reliance on past authority, and called for a new method of inquiry based on controlled observation.

The greatest intellectual figure of the medieval era was Saint Thomas Aquinas, a Dominican monk who studied under Albertus Magnus, following him to Cologne in 1248. Aquinas combined Aristotelian science and Augustinian theology into a comprehensive system of thought that later became the authoritative philosophy of the Roman Catholic Church. He wrote on every known subject in philosophy and science, and his major works, the Summa Theologica and Summa Contra Gentiles, in which he presents a persuasive and systematic structure of ideas, still constitute a powerful influence on Western thought. His writings reflect the renewed interest of his time in reason, nature, and worldly happiness, together with his religious faith and concern for salvation.

Aquinas argued against the Averroists that the truths of faith and the truths of reason cannot conflict but rather apply to different realms. The truths of natural science and philosophy are discovered by reasoning from facts of experience, whereas the tenets of revealed religion, the doctrine of the Trinity, the creation of the world, and other articles of Christian dogma are beyond rational comprehension, although not inconsistent with reason, and must be accepted on faith. The metaphysics, theory of knowledge, ethics, and politics of Aquinas were derived mainly from Aristotle, but he added the Augustinian virtues of faith, hope, and charity and the goal of eternal salvation through grace to Aristotle's naturalistic ethics with its goal of worldly happiness.