This dissertation is a defence of metaphysical ethical naturalism according to which there is a moral reality which is part of the natural world. The implication of this view is that moral properties, such as moral goodness, justice, compassion and so forth are part of the natural world, and inquiries concerning these moral entities are conducted in similar empirical ways of reasoning to that in which scientific inquiries are conducted. I defend metaphysical ethical naturalism by a variety of explanationist argument in the tradition of Cornell realism. I examine preceding proposals for this argument, and focus on one version of it, which I call ‘the abductive argument for moral realism’. Although there was a suggestion about the abductive argument, the argument has not been discussed enough in the literature. This dissertation is a defence and discussion about the abductive argument which has not been properly examined. The defence of the argument requires the examination of how first-order ethical theory can be developed in the similar empirical ways scientific theories are developed. This will be an attempt to show the analogy between scientific inquiries and ethical inquiries. Describing the analogy between science and ethics, I will argue that the analogy can be best explained in terms of the approximate truth of normative theory which implies the existence of mind-independent natural moral properties.