I. The Open Question Argument: First Version
ETHICAL NATURALISM IS AS FOLLOWS:
(EN1) Each moral property is identical to some natural property.
(EN2) Each moral term (e.g., good) refers to some natural property and has that property as its semantic content.
The Open Question Argument is alleged to show that if moral realism is true, then each moral property must be identical with some non-natural property and that each moral term like "good" does not have some natural property as its content.
Consider the following question schemas:
(Q1) Helping a starving child is good, but is helping a starving child N?
(Q2) Helping a starving child is good, but is helping a starving child good?
Let N range over natural properties. The assumption is that for any candidate natural property for N, the question yielded by filling in the variable in (Q1) would always be open, whereas the question yielded by filling in the variable in (Q2) would always be closed.
What it is for a question to be open and closed is difficult to explain, and it might help if we had an example. So let's substitute pleasant-ness for N. We get the following two questions by filling in the variables from (Q1) and (Q2):
(Q1A) Helping a starving child is good, but is helping a starving child pleasant?
(Q2A) Helping a starving child is good, but is helping a starving child good?
Q1A and Q2A differ in cognitive significance. A reasonable person might sensibly ask Q1A. But it seems that a person could not sensibly ask Q2A. Q2A is closed in the sense that once you have made a judgment that something is good, you already have your answer to the question "Is it good?" Q1A is open in the sense that, having made a judgment about the goodness of helping the poor, you do not necessarily have an answer to that question.
The problem for ethical naturalism is that if good-ness is identical to some natural property and "good" refers to some natural property, then if Q2 questions are closed, then Q1 questions should also be closed (once you plug in the appropriate natural property for N). However, it seems that the two could come apart. Q1A and Q2A should be the same question if ethical naturalism is true, but they are not. You have an answer to one, but not the other.
The Open Question Argument
(1) If ethical naturalism is true, then there is some natural property N, such that schemas Q1 and Q2 yield the same questions.
(2) There is no natural property N such that schemas Q1 and Q2 yield the same questions.
(3) Therefore, ethical naturalism is not true.
Ethical naturalism is committed to (1). If good is identical to some natural property N and the term "good" just means N, then any sentence using a term T other than good that means N will express the same proposition as a sentence that substitutes T with good. A substitution will yield the same proposition; a substitution of terms would also yield the same question.
But in the case above we are supposed to think that they are not the same question. They are not the same question because the questions have different properties; the property being-open is had by Q1A (and not Q2A). The property being-closed is had by Q2A (and not Q1A). That is the motivation for (2). We are supposed to see that this would be true no matter what natural property we might try to identify with good; so there is no natural property N such that Q1 and Q2 would yield the same questions. For any natural property, there would be this difference in cognitive significance. (1)
In this section I will lay out what I think is one of the best responses to the Open Question Argument. As we shall see, the Open Question Argument is basically a kind of Frege Puzzle. Frege's Puzzles are problems for a theory in philosophy of language called Millianism. …