John Scotus Eurigena developed a somewhat original philosophical theory of the levels of Nature, which divides all existing being into four categories: God, universals, creatures and the Divine Nature. It is God's grace, which actively makes it possible for humans to return to God and for the whole Universe to be ultimately united with God, thus comprising the Divine Nature, in which every being will be united with each other in God and will ultimately lose its own individual properties. On the part of man, he may reach a complete unity with God in the state of 'theosis' (i.e. deification), which may be attained through the co-operation between a man and the grace of God. Moreover, Eriugena held that it is the faculty of human reason, which has the primary responsibility for pursuing God and for attaining the ultimate unity with God in the Divine Nature.