Answer
Eriugena's work is considered an achievement of ancient philosophy because it is heavily influenced by Neoplatonism, a school of philosophy that was developed in the 3rd century AD. Neoplatonism was based on the work of Plato, and it emphasized the importance of reason and logic in understanding the universe. Eriugena's work is also influenced by the Christian tradition, but he uses Neoplatonic ideas to interpret Christian doctrines.