The Naturalistic Fallacy and the Intuitionist Ethical Theories ― Lyou, Ji-Han ― The naturalism and intuitionism had been developed in competition with each other since 17th century. But G. E. Moore exposed that all forms of naturalism committed 'naturalistic fallacy' by means of 'open question argument'. Moore's criticism against naturalism inspired intuitionism to new endeavors. All forms of intuitionism are based on four theses. 1. The Nonnaturalism Thesis : Moral terms cannot be defined by nonmoral (natural) terms because moral terms refer to nonnatural properties. 2. The Moral Realism Thesis : There are sui generis moral properties or self-evident moral principles which cannot be reduced to natural properties or non-moral principles. 3. The Cognitivism Thesis : Moral Statements are either true or false and they can be known. 4. The Intuition Thesis : Moral truths are discovered by the intuition and they are self-evident upon reflection. While intuitionism doesn't commit the naturalistic fallacy owing to The Nonnaturalism Thesis, it is criticized for both The Moral Realism Thesis and The Intuition Thesis. Intuitionism has difficulty in demonstrating The Moral Realism Thesis because of the ontological queerness and mysteriousness of nonnatural properties. Also it is blamed for having the difficulty in justifying 'knowledge by intuition' because there are no accredited routes to the knowledge. Finally, intuitionism collapses into relativism because it cannot give the objective and rational methods except intuition which are capable of resolving the conflict of intuitions in moral disagreements. ※ Key Words : naturalism, intuitionism, naturalistic fallacy, nonnaturalism, moral realism, cognitivism.