 Mackie replies that if God has mere metaphysical or factual necessity, God’s existence is logically contingent, such that some reason is required for God’s own existence (Mackie 1982: 84). As Swinburne notes, God is a logically contingent being, and so could have not-existed (2004: 79, 148). Why, then, does God exist? The PSR can be applied to the necessary being.