Augustine had no formal education in philosophy, but nevertheless was a natural philosopher with varied interests. His use of philosophy in his theological writings had an impact and an abiding influence on Western philosophical thought.
The Ten Main Contributions of Augustine to Philosophy.
1. Theory of Time: In the Confessions Book 11 Augustine developed a very provocative concept of time.
2. Learning Language: Augustine attempted to explain how small children learn and express language.
3. Faith Seeking Understanding: In his Sermon 43.7, 9 Augustine asserted: Crede, ut intelligas ("Believe in order that you may understand").
4. Ontological Argument: The writings of Augustine also influenced the subsequent formulation by Saint Anselm of the ontological argument for the existence of God.
5. Refutation of Scepticism: The statement of trench philosopher, René Descartes (1596-1650), cogito ergo sum ("I think, therefore I am") is derived from dubito ergo sum ("I doubt, therefore I am") and si fallor sum ("If I am deceived, I am") by Augustine.
6. Proof of the existence of God from Eternal Truths: Augustine argued that the human mind apprehends universal, objective, unchanging, and necessary truths that are superior to the human mind itself. Thus an eternal God exists to explain these eternal truths.
7. Response to the Problem of Evil: Augustine argued that while evil is real it is not a substance or a "stuff." Rather, evil is an absence of goodness.
8. Divine Illumination: Augustine developed an epistemology (theory of knowledge) known as divine illumination. Human knowledge is thus directly dependent upon God.
9. Creation ex nihilo: Augustine vigorously argued that God created the world ex nihilo (creation "out of nothing" or "from nothing"). Certain points of the thought of Augustine 1,500 years ago are very consistent with the modern "big bang" theory within cosmology.
10. The Examined Self: In his Confessions, Augustine was one of the first to write in depth about the self, particularly in relation to God.
While a student, Augustine discovered philosophy. He was extremely impressed by a book he read by the great Roman pagan author, Cicero. In his book called Hortensius, Cicero argued that only the philosopher's life is worth living. Augustine later stated that it was this book that had set him on his own spiritual quest to find God, and led to his conversion. This early Christian philosophy of Augustine was largely his rewriting in a Christian context ideas taken from Plato.
The main change that Christian principles brought to the Platonic way of thinking was a moderation of the Stoicism and of the theory of justice of the Roman world, and an emphasis on the role of the state in applying mercy as a moral example. The comment that "Augustine baptised Plato" is too simplistic. Certainly, Augustine took much from Plato and from Neo-Platonism and selectively used those thoughts therein that aided him more effectively to explain and to communicate Christianity.
The Oxford History of Western Philosophy summarizes the tremendous influence of Augustine on Western intellectual history as follows: "It is arguable that Augustine is the most influential philosopher who ever lived. His authority has been felt much more broadly, and for a much longer time, than Aristotle's, whose role in the Middle Ages was comparatively minor until rather late. As for Plato, for a long time much of his influence was felt mainly through the writings of Augustine.For more than a millennium after his death, Augustine was an authority who simply had to be accommodated. He shaped medieval thought as no one else did. Moreover, his influence did not end in the Middle Ages. Throughout the Protestant Reformation, appeals to the authority of Augustine were common place on all sides. His theory of illumination lives on in Malebrache and in the "light of nature" of Rene Descartes. His approach to the problem of evil and to human free will is still widely held today. His force was and is still felt not just in philosophy but also in theology, popular religion, and political thought." (Anthony Kenny, ed. The Oxford History of Western Philosophy (Oxford: Oxford University, 1994), pages 57-58.
The Mark of Augustine on Philosophy. By Eugene Portalie in the first edition of the Catholic Encyclopedia. Difficult reading. http://www.newadvent.org/cathen/02091a.htm#III