time of Augustus. Pausanias, who lived in the 2nd century AD, was also a geographer. His Description of Greece is a travel guide describing the geography and mythic history of Greece during the second century. The book takes the form of a tour of Greece, starting in Athens and ending in Naupactus. The scientist of the Roman period who had the greatest influence on later generations was undoubtedly the astronomer Ptolemy. He lived during the 2nd century AD, though little is known of his life. His masterpiece, originally entitled The Mathematical Collection, has come to the present under the title Almagest, as it was translated by Arab astronomers with that title. It was Ptolemy who devised a detailed description of an Earth-centered universe, a notion that dominated astronomical thinking for more than 1,300 years. The Ptolemaic view of the universe endured until Copernicus, Galileo, Kepler, and other early modern astronomers replaced it with heliocentrism. Philosophy Epictetus ( 55 AD – 135 AD) was associated with the moral philosophy of the Stoics. His teachings were collected by his pupil Arrian in the Discourses and the Encheiridion (Manual of Study). Diogenes Laërtius, who lived in the third century AD, wrote Lives and Opinions of Eminent Philosophers, a voluminous collection of biographies of nearly every Greek philosopher who ever lived. Unfortunately, Diogenes Laërtius often fails to cite his sources and many modern historians consider his testimony unreliable. Nonetheless, in spite of this, he remains the only available source on the lives of many early Greek philosophers. His book is not entirely without merit; it does preserve a tremendous wealth of information that otherwise would not have been preserved. His biography of Epicurus, for instance, is of particularly high quality and contains three lengthy letters attributed to Epicurus himself, at least two of which are generally agreed to be authentic. Another major philosopher of his period was Plotinus. He transformed Plato's philosophy into a school called Neoplatonism. His Enneads had a wide-ranging influence on European thought until at least the seventeenth century. Plotinus's philosophy mainly revolved around the concepts of nous, psyche, and the "One." After the rise of Christianity, many of the most important philosophers were Christians. The second-century Christian apologist Justin Martyr, who wrote exclusively in Greek, made extensive use of ideas from Greek philosophy, especially Platonism. Origen of Alexandria, the founder of Christian theology, also made extensive use of ideas from