| Hume is to supposed to have argued to we can't get an "ought" from an "is." Moral conclusions can only, it is argued, follow from an argument that includes at least one moral premise (one "ought").
Let's suppose that I've reached the conclusion that my jeans are blue. What kinds of premises would have to be present in the argument from which that conclusion follows? The conclusion concerns a quality of blueness as distinguished from non-blueness.
If a moral conclusion requires a moral premise, does a blueness conclusion require a blueness premise? If not, why not?
I suppose Hume might have argued that preception of color and of immorality are different kinds of perceptions. Some (maybe not Hume) would say that moral perceptions are subjective in that they describe the perceiver's state of mind and not an objective quality outside the mind. Says who? Why can't I claim that color is all in the mind? Can anyone prove to me it isn't?
"Ahh," you might say, "but people most often agree that, for example, the sky is a lighter version of the same color as David's jeans--that they are both blue. Doesn't that suggest that the perception may be of something truly oblective?" However, I could counter that about the same level of agreement could found among people who are shown are child being sexually abused that what they see is wicked.
I perceive wickedness; I perceive blueness. Show me an argument that demonstrates that one of these perceptions must be more subjective than the other.
"Ahh," you might counter at this point, "but color is explainable in terms of wavelengths of electromagnetic radiation. What physical quantity do you perceive when you perceive wickedness?"
I would counter that, while color may be a matter of one physical quality (i.e., the frequency of EM radiation), many other perceptions are much more complicated. For example, many qualites have been identified as affecting human perceptions of feminine beauty. However, identifying these qualities doesn't explain the perception to which they contribute. It doesn't tell us what beauty is.
Are we willing to give up the concept of beauty and just say that certain faces have certain measurable qualities which people usually react to in certain ways?
Furthermore, if I can verify experimentally what qualities contribute to the perception of beauty, is it not likely that I can find qualites in human action that contribute to the perception of wickedness?
Even if I can reduce beauty to the physical qualities that contribute to the perception, does that in any way imply that it's irrational of me to enjoy feminine beauty when I see it? Similarly, if I can identify certain objective qualities that contribute to the perception of wickedness, does that in any way imply that I should not oppose wickedness when I perceive it? Hume seems to say that it does. If an "is" can't lead to an "ought," then I need some need an axiom that says I ought to oppose wickedness, but would that axiom be tautologous? Would it be similar to saying beautiful women are beautiful?
I actually believe that nature has given me the ability to perceive many qualities, and that color, beauty, and wickedness are among them. No one has yet argued me out of this belief.