This book has two main strands of ideas that encompass not only the beginnings of philosophy, but also the foundation of Western civilization and much of what we take for granted in our mental outlook in the modern world. The two strands are: ancient – Plato, Aristotle, Lucretius, Sextus Empiricus and Plotinus; and medieval – Augustine, Anselm, Aquinas, Duns Scotus and Ockham. These comprise, roughly speaking, two thousand-year stretches from respectively 600 BCE to 300 CE, and 300 CE to 1600 CE. Without knowledge of these thinkers an understanding of the Western world would be a poor thing indeed. The ancient strand, which extends into the Roman period, is pre-Christian in its dominant ideas and contrasts with the medieval strand when Christian ideas prevailed. However, Christian European philosophy does not mark a break with ancient philosophy, but rather selectively incorporates it within the overarching Christian worldview. This is not a book of history that is concerned with the causes and influences of ideas whatever their merit, but a book of philosophy that is concerned with outstanding ideas and the quality of the arguments for them.

We begin with Plato (427–347 BCE) because philosophy proper begins with Plato. Plato's work marks perhaps the most important turning point in the development of mankind. For the first time in human history, when considering what is true, what we should believe, how we ought to live, the answers are advocated not on the basis of ideas found in supposedly holy books, or the word of great authorities, or the mere length of time an idea has been around.