Propositions are, roughly, what meaningful sentences are supposed to express. Different sentences, in different languages, can (it is often thought) express the same proposition: 'snow is white' and 'schnee ist weiss' both express the proposition that snow is white. A common assumption among philosophers who use this jargon is that propositions, properly speaking, are what are true or false (what bear truth values). So if an ethical sentence does express a proposition, then the sentence expresses something that can be true or false.
To get a better idea of what it means to express a proposition, compare this to something that does not express a proposition. Suppose someone minding a convenience store sees a thief pick up a candy bar and run. The storekeeper manages to exclaim, "Hey!" In this case, "Hey!" does not express a proposition. Among the things that the ejaculation does not express are, "that's a thief there"; "that thief is getting away"; or "that thief really annoys me." The storekeeper is saying anything at all, really, at least nothing that can be true or false. So it is not a proposition that the storekeeper is expressing. Perhaps it is an emotional state that is being expressed. The storekeeper is surprised and angered, and expresses those emotions by saying, "Hey!"
It is an essential part of ethical naturalism that ethical sentences do express propositions. They are not just emotional outbursts, as though we were saying, "Hey!" or "Yay for Mary!" They are actually expressing propositions that can be true or false. Derivatively, the naturalist would say that ethical sentences themselves are either true or false.
For example, an ethical naturalist might hold that it can be true or false that Mary is a good person; it can be true or false that stealing and lying are always wrong. On the other hand, if one believes the sentence, "Mary is a good person," cannot be either true or false, then one is not a naturalist.
Notice, too, that if we say that ethical sentences merely express emotions (they "do" no more than that: see speech act), as though they they were just exclamations like "Hey!" and "Yay for Mary!", then we cannot also think that ethical sentences are true or false. It would be nonsense to say, "It's true that 'Hey!'" or "It's false that 'Yay for Mary!'" Mere expressions of emotion might be appropriate ("apt" is the jargon bandied here) or inappropriate, but not true or false.
wikipedia.org dumped 2003-03-17 with terodump