Numerous variants of the theory have been presented: historically, figures including Saint Augustine, Duns Scotus, William of Ockham and Søren Kierkegaard have presented various versions of divine command theory; more recently, Robert Merrihew Adams has proposed a "modified divine command theory" based on the omnibenevolence of God in which morality is linked to human conceptions of right and wrong. Paul Copan has argued in favour of the theory from a Christian viewpoint, and Linda Trinkaus Zagzebski's divine motivation theory proposes that God's motivations, rather than commands, are the source of morality.