Pseudo-Dionysius is the name given to an anonymous Christian theologian and philosopher who lived in the late 5th to early 6th century. He is best known for his writings on mysticism and the nature of God, particularly in works like The Divine Names and The Mystical Theology. His ideas were heavily influenced by Neoplatonism, a philosophical system that emphasizes the existence of a single source of all reality. His writings introduced the concept of the "via negativa," which suggests that God can only be described by what He is not, rather than what He is. This approach has had a lasting impact on Christian mysticism and theology, influencing later thinkers such as Thomas Aquinas and Meister Eckhart.