While the influence of classical pagan philosophy was crucial for the development of medieval philosophy, it is likewise crucial that until the twelfth and thirteenth centuries almost all the original Greek texts were lost to the Latin West, so that they exerted their influence only indirectly. They were “lost” not in the sense that the texts were simply unavailable but in the sense that very few people could read them, since they were written in the wrong language. As the Western Roman Empire gradually disintegrated, the knowledge of Greek all but disappeared. Boethius (c. 480–545/526) was still fluent in Greek, but he recognized the need for translations even in his own day; after him Greek was effectively a dead language in the West. There were still some pockets of Greek literacy, especially around such figures as Isidore of Seville and the Venerable Bede, preserving and transmitting ideas of ancient learning, but making little impact on medieval philosophical thought.
In the case of Plato, the Middle Ages for all practical purposes had only the first part of the Timaeus (to 53c), hardly a typical Platonic dialogue, in a translation and commentary by a certain Calcidius (or Chalcidius)… This state of affairs lasted until the Renaissance, when Marsilio Ficino (1433–99) translated and commented on the complete works of Plato.
For Aristotle, the Middle Ages were in somewhat better shape. Marius Victorinus translated the Categories and On Interpretation. A little over a century later, the logical works in general, except perhaps for the Posterior Analytics, were translated by Boethius, c. 510–12, but only his translations of the Categories and On Interpretation ever got into general circulation before the twelfth century. The rest of Aristotle was eventually translated into Latin, but only much later, from about the middle of the twelfth century.
Still, while it is important to emphasize this absence of primary texts of Greek philosophy in the Latin Middle Ages, it is also important to recognize that the medievals knew a good deal about Greek philosophy anyway. They got their information from (1) some of the Latin patristic authors, like Tertullian, Ambrose, and Boethius, who wrote before the knowledge of Greek effectively disappeared in the West, and who often discuss classical Greek doctrines in some detail; and (2) certain Latin pagan authors such as Cicero and Seneca, who give us (and gave the medievals) a great deal of information about Greek philosophy.
During the first part of the Middle Ages, Platonic and neo-Platonic influences dominated philosophical thinking. “Plato himself does not appear at all, but Platonism is everywhere,” as Gilson has said. (Gilson [1955], p. 144.) This situation prevailed until the recovery of Aristotle in the twelfth and thirteenth centuries.