The Medieval Period

8.1 Between Ancient and Modern

The influence of Plato and Aristotle seems to have run its course at the dawn of the medieval period (roughly 500–1500 CE). After the fall of the Roman Empire and with the rise of Christianity, a master, censor, and patron of philosophy emerged.

The relationship between the Church and philosophy was one in which the latter was needed to make sense of the former’s teaching or shore up the relationship between faith and reason. Consequently, there is much overlap between theology and philosophy in this period.

8.2 Augustine

Trained in rhetoric, but impassioned by philosophy, Augustine was originally interested in the Manichean explanation of evil. Later intrigued by Neoplatonism for the way it elucidated features of Christianity, Augustine went on to enlist his interest in understanding his faith across a wide range of philosophical topics, from free will to ontology.


Augustine argues against the skeptics in much the same way as does Descartes: If he is mistaken, as the skeptics would have it, then he exists. Moreover, since he is certain of this fact, he knows it. Lastly, he is glad of it.

To these truths are added mathematical and logical truths—in short, necessary truths. From necessary truth, he argues for the existence of God.

The Hierarchy of Being

The “Great Chain of Being” follows Plato’s own hierarchy of being but replaces the Form of the Good with God. Augustine’s great chain of being moves from the least (physical, imperfect) to the most real (Being, or God).

Good and Evil

Goodness is correlated with being. From this, Augustine concludes that nothing that exists can be evil. So, all of God’s creations are good. All evil is only a privation of good.

8.3 Anselm and Aquinas


Best known for his version of the ontological argument, Anselm focuses on arguing for the necessary reality of God from the concept of God. Since God is, by definition, the greatest possible being, it is impossible for him to exist only conceptually.

Anselm’s argument is not without its critics. Most notable among them is Gaunilo, who objects to the idea that perfection entails existence. Were that a plausible line of reasoning, Gaunilo argues, then we could infer the existence of a perfect island. Anselm’s response is that the concept of perfection applies exclusively to God.


Aquinas’s fusion of Aristotelian philosophy with Christian doctrine resulted in, among other things, some of the best-known arguments for God’s existence. In his monumental Summa Theologica, Aquinas presents his Five Ways of proving God’s existence, the first two of which both start from the empirical fact of the universe’s existence:

  1. The argument from motion: Things move or change, and so must have been set in motion. If things moving other things went on ad infinitum, there would not be something that set the series into motion. So, there must be a first mover (God).
  2. The first cause argument: We observe that things have causes. From this, we can infer a first cause of the universe (God), because it is an absurdity to suppose that causes can go on ad infinitum.

Aquinas also contributed groundbreaking work in natural law theory. On his view, right actions are those that conform to moral standards discerned in nature through human reason. Humans achieve their highest good when they follow their true, natural inclinations leading toward their goals.

This Aristotelian approach to morality does not mean that performing bad actions to achieve a good effect (en route to one’s highest goal) is acceptable. According to the doctrine of double effect, good actions that yield bad results are acceptable, but not the converse.

8.4 Avicenna and Maimonides


Persian philosopher and physician Ibn Sina (Avicenna) developed a systematic and comprehensive metaphysics, which combined his own interpretation of Aristotelian concepts with Neoplatonism, and which reconciled these with Islamic theology.


The Jewish legal scholar, philosopher, and physician Moses Maimonides attempted to comprehensively reconcile religious tradition with philosophy (especially Aristotle).

8.5 Hildegard of Bingen

The 12th-century philosopher-theologian Hildegard of Bingen was one of the very few women to break out of intellectual and professional conditions almost entirely inhospitable to women. One of the first religious mystics in the West, Hildegard contributed to theories in ethics, natural science, and medicine.

8.6 William of Ockham

Ockham is best known for his principle of parsimony (Ockham’s razor), which asserts that, in devising explanations or theories to explain a phenomenon, we should prefer the simpler theory over the more complex.

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