The Divine Names works through the titles scripture gives to God, goodness, being, life, wisdom, and shows in each case how the word applies and how it fails. The Mystical Theology, which is a few pages long, then removes them. God is not soul, not intellect, not number, not order, not greatness, not equality, not eternity, not divinity, not goodness, not spirit as we understand spirit; and finally the denials themselves must be denied, because negation is still a statement made by a mind. What is left is what he calls the divine darkness, into which Moses enters on the mountain, a darkness which is dark by excess of light.
That is the classical statement of the negative way, and everything in the Western and Eastern mystical traditions that proceeds by unknowing runs back to these pages: Eriugena, the author of the Cloud of Unknowing, Eckhart, John of the Cross, and by another route Maximus the Confessor and the whole Byzantine tradition.
The other half of the corpus is the opposite in temper and equally influential. The Celestial Hierarchy arranges the angels in nine ranks in three triads, seraphim and cherubim and thrones at the summit, and the Ecclesiastical Hierarchy sets the church's orders in a corresponding pattern, so that being flows down through ranks and returns upward through them. The nine choirs of angels in Western art and liturgy are his, and so is a great deal of medieval thinking about ordered mediation.