Substantive ethical naturalists believe that the ethical is natural, that is, that ethical properties are natural properties. Strong, or reductive, ethical naturalists hold that there is an interesting further question of which natural properties the ethical ones are, just as there is an interesting question of which chemical property water is (Railton 1986, Jackson 1997). Weak ethical naturalists deny this; some hold that ethical properties, though natural, are irreducible (Boyd 1988), while others are in the business of revising moral language (Brandt 1979).

Moore's open-question argument was advanced against strong ethical naturalism, which he claimed committed the naturalistic fallacy. The basic idea of the argument is that we can test cognitive significance to test claims of property identity. So, for example, if good just is pleasure, then "Pleasure is good" should mean the same as "Good is good," and be equally informative to speakers who understand the two...