Moral realism is the meta-ethical view that moral statements are statements of fact that describe inherent properties of the universe. It goes hand in hand with the epistemological view that humans are capable of knowing these moral properties of the universe. Moral realism comes in two varieties: naturalism and non-naturalism. Naturalism is the view that moral properties are reducible to one or more features of nature, such as pleasure and pain. Non-naturalism is the view that moral properties exist in their own right independent of any features of nature, so that “goodness” and “badness” are not reducible to any discernible feature of nature.
There are two major problems with any theory of moral realism. First, there is the epistemological problem. With respect to naturalism, this is the “is-ought” problem, or the impossibility of stating that people ought to or have a duty to behave in a certain way based on any discernible facts about the universe. With respect to non-naturalism, there are two main problems: first, there is the problem of determining which purported means of knowledge relied upon are reliable or, stated a different way, of proving that the purported means of knowledge relied upon are themselves reliable; second, there is the impossibility of adjudicating between differing views arrived at by the same purported means of knowledge.