I have gotten a number of comments recently concerning the distinction between a descriptive claim and a prescriptive claim.
For example, this relates to Luke's question:
Yes, but then you are only describing the reasons for which people actually do things. This is a descriptive theory of intentional action. How do you get from descriptive to prescriptive?
Actually, I describe the reasons people have for doing things. What people actually do is a combination of their desires and beliefs. If a person drinks a glass of water that happens to be poisoned, I describe why she actually drank the water (she was thirsty and she believed the water was not poisoned).
However, I can also prescribe that she not drink the water (because the relationship between the state of affairs that will result from drinking the water and her desires is not what she believes it will be).
At the root of it, if somebody wants to assert that there is some sort of objective moral truth (as I do), they had better be prepared to defend moral claims as being both (at the same time) descriptive and prescriptive. Insofar as it is a moral claim, it must be prescriptive. Insofar as it is objectively true, it must be descriptive.
I do need to clarify that by objective in this sense I do not mean independent of any mental state. I am talking about objective in the sense of refers to something in the real world. Mental states are a part of the real world, as are relationships between mental states and states of affairs in the world.
Relationships between mental states and sates of affairs in the world are clearly not objective in the sense that they are independent of any mental state. In the absence of mental states, they would cease to exist. However, they are still objective in the sense that you can point to something in the real world and say, "That's it," even if the thing you are pointing to is at least partially made up of mental states.
It does not follow that, just because a claim is descriptive that it cannot be prescriptive. If you begin with this assumption, then you are begging the question against any type of moral realism. This is a claim that has to be defended. It cannot be assumed. So, "Hey, your claim here is descriptive," is not proof that one is not talking about values (prescriptions). One needs to claim something more. One needs to claim, "Hey, your claim here has nothing to do with reasons for action."
If this is true, then one has proved that the claim being challenged has nothing to do with value.
Value exists as a relationship between reasons for action that exist and states of affairs.
True value claims are descriptive in that they are concerned with actual relationships between reasons for action that exist and states of affairs. The claim points to something real – even if that which is real happens to be a mental state (i.e., a desire).
True value claims are prescriptive in that they are concerned with actual relationships between reasons for action that exist and states of affairs. The claim points to a reason for doing something – an end or a goal – and identifies the action as an effective way to realize that goal or end state.
True value claims are descriptive and prescriptive at the same time.
Now, I also hold that desires are the only reasons for action that exist. Divine commands, intrinsic values, categorical imperatives . . . if any of these reasons for action actually existed, they would be relevant to moral claims. However, they do not exist.
Consequently, any claim of the form, "Here is a reason for action that the agent not drink the water – because God commands that no water be drank on Wednesdays and today is Wednesday." This type of claim is false. No such reason for action (or, in this case, inaction) exists.
Or, any claim of the form, "There is a reason for action that says that the woman ought not to drink the water in that women drinking water on Thursdays is intrinsically bad, and today is Thursday," is also false. If intrinsic values existed they would be morally relevant reasons for action. However, they do not exist. Therefore, any claim that refers to an intrinsic value is a false claim and does not describe a reason for action that exists.
Any claim that refers to a desire refers to a reason for action that exists. These claims can be true. They can also be false – if they refer to a desire that does not exist, or they assert a relationship between a state of affairs and a desire that is not true.
Of course, we also have the power to make claims about relationships between states of affairs and desires that people generally have reason to promote or inhibit. These types of claims are also real.
If an action bears a particular relationship to desires that people generally have reason to promote, then they have reasons for action to bring the social tools of praise and reward to bear to do the promoting. If an action bears a particular relationship to desires that people generally have reason to inhibit, then people have reason to bring the social tools of condemnation and punishment to bear to do the inhibiting.
In both cases, if you are describing a relationship between an object of evaluation and reasons for action that exist, you are also (at the same time) prescribing that object of evaluation in virtue of its relationship to reasons for action that exist.
Now, not all descriptive claims are prescriptive. This is true because not all descriptive claims refer to reasons for action. However, a descriptive claim that describes a relationship between an object of evaluation and a reason for action is, at the same time, prescriptive.
Evidence of descriptivity is not proof against prescriptivity.