  Aristotle also contributed to acceptance of the full DDI. For many medieval theists accepted Aristotle's case for God's existence. Aristotle's Physics reasoned that if change occurs, it has a final source, an eternally unchanged changer. Aristotle's De Caelo added that something is eternally unchanged only if unchangeable. Later theists thought the role of first cause of change too lofty not to be God's. Writers who took Aristotle's argument or its descendants to prove God's existence found themselves committed to DDI.