We have to decide in what direction to educate our desires, how to order a variety of impulses, felt needs, emotions and purposes. Hence those rules which enable us to decide between the claims of, and so to order, our desires—including the rules of morality—cannot themselves be derived from or justified by reference to the desires among which they have to arbitrate.
This is the assumption that reason is entirely separate from 'desirs' AND that somehow it must preferentially rule them
