One of my favorite questions to think about...
Are there such things as 'objective' moral values? That is, can we say anything is 'good' or 'bad' without making reference to some subjective opinion? Can a thing or action be called 'good' in the same way that an apple can be called 'red'? If so, what is the source of this 'goodness' or 'badness'? Are they natural properties, like shape or color, or do they derive from some supernatural source (i.e., God)? How do we gain knowledge of these 'moral' properties?
Suppose, for argument's sake, that moral properties are not supernatural—thus, properties like 'goodness' and 'badness' must either arise from natural facts about the world, such as (for example) pleasure or pain, or they must be no more than expressions of our subjective opinions. That is, saying "Murder is wrong" is equivalent to saying something "I don't like murder" or "You should not commit murder". Obviously, these interpretations don't correspond with what we think we mean when we make moral judgments. Furthermore, if we say that values are simply subjective preferences, there is no way for us to make moral judgments—even a despicable act like the Holocaust would be one we merely find distasteful: we would have no grounds for calling it evil.
Presumably, we want to be able to make moral judgments against those things we find abhorrent, and in favor of those we value. So how can we derive moral facts from so-called natural facts? We could adopt the hedonist thesis, that the 'Good' is simply whatever gives us the most pleasure—although, as I noted in an earlier post about the Experience Machine, that's probably false, and we usually don't think of 'pleasure' and 'good' as simple synonyms. The question remains: What objective basis is there for defining 'good' and 'bad'?
Must we find this basis in the supernatural—in, say, God's moral laws, or something like the Platonic forms? Or must we concede that 'good' and 'bad' simply reflect contingent historical, cultural, and/or personal preferences?