[Projoy] Self-interest is at work in stepping out of the path of a car, but that doesn't make it subjective in any useful sense. Everyone will step out of the way of that car, except only children who have not yet learned that it is dangerous, the drug-addled, and the suicidally depressed. This is only subjective to the degree that every thought we have is subjective. What is objective is that the car will kill you if you don't get out of the way. Opinions and wishes are irrelevant. Reality is like the Terminator: you can't reason with it, you can't argue with it. You only have power to choose your actions, not their consequences.
       (Repeat earlier caveat that I'm taking this horse out for a ride to test its legs.)
       Thus it is with morality. The enlightened do good and avoid evil, because, from their point of view, the first person that good acts benefit and evil acts harm is oneself, consequences as undeniable as traffic. It applies to everything from not mugging old ladies for their pension money to getting out of bed in the morning. The idea has a long pedigree, e.g. Socrates and the Buddhist canon (and in a debased form in the Religions Of The Book, where all the consequences are deferred to a supposed later life, despite the statement by one of their prophets that "the kingdom of heaven is within you"). It doesn't give instant answers to all questions, any more than the laws of physics will immediately tell you how to build a kilometer-tall skyscraper.

[Bm] You give two arguments that undercut each other: that there are no universally accepted moral principles, and that universally accepted moral principles arise for evolutionary reasons. You can't have it both ways, but I'll ride this horse over both of them anyway.
       No scientific truth is universally accepted either, if you define "universally" narrowly enough. There are, nonetheless, substantial areas of agreement on moral issues that can be found throughout all civilisations. That doesn't prove they are true (as Lewis himself says), merely that they exist. Perceiving their truth is a separate matter.
       I could take the evolutionary explanation as evidence on my side -- the consequences of right and wrong action are exactly your presumed evolutionary pressures. Not getting run over is also evolutionarily adaptive, but that does not mean that personal decisions and the laws of physics have nothing to do with it.
       How do you perceive the utility of the Way? If it is useful to follow the Way, what is it useful for? And what in turn is that useful for? Utility offers no foundation. Likewise rationality. Look hard enough, and all attempts to find foundations lead only to an infinite regress. Ideas can only justify ideas in terms of other ideas. Morality is about actions, and actions cannot be deduced from thoughts, any more than an ought from an is.
       Hume also claimed not to see causation, but he wouldn't have survived to write his books if he hadn't dodged horse-drawn carts now and then.

[Raak] Maybe Hume was just very, very lucky.

[Raak] Your supposition that evil acts harm the doer is where it all falls down for me. Sort of repeating Bm's point, but a car will run people down according to the laws of physics, which can be reliably shown to operate universally (at least as far as we can observe), but mugging someone for their pension cannot be shown universally to have negative effects for the individual (unless you start invoking some unknowable afterlife or karma or whatever). In other words, some people get away with breaking moral rules, in just the way that a car can't get away with braking the rules of physics in order to save a life. There is merely a certain probability that a system of law exists that will make mugging disadvantageous to the perpetrator. This is not 100% (in fact taking human history as a whole, I'm tempted to speculate it is less than 50%). Is there any real evidence whatsoever to the contrary?

You might come back with the argument that the fact people get away with breaking moral rules is irrelevant to the truth of those rules, but if so then the truth of those rules is irrelevant to us if we are searching for objective standards, since the rules cannot be shown to have reliable consequences, and therefore do not admit of predictions.

I also think that rational suicide is a very interesting example in the objective/subjective debate, because it's always struck me as a very interesting example of higher-order thinking successfully overruling all the lower-order evolutionary thinking with which we come pre-installed. If our sense of objective morality rests on self-preservation, why would it admit of contradiction in this way?

Raak] (Following on from Breadmaster's reply) It's also the case that all of these moral traditions arose in societies whose rulers did not follow them, yet all of these moral traditions preached acceptance of those rulers. "Thou shalt not kill" actually become "Thou shalt not kill unless thou art the state". If these are absolute moralities, shouldn't the rulers obey them? And if the rulers are immoral, shouldn't the moral organise to get rid of them and impose the rule of the moral?
My second objection is that I don't see what makes these an objective rule-set, as opposed to the many other moral precepts you could have instead (for example, sexual equality, environmentalism).
Lastly, illustrating Projoy's point, I'm reminded by the weather of the Belloc (?) poem:
The rain it falleth on the just
And on the unjust fella
But more upon the just, because
The unjust stole the just's umbrella
One of the lessons of experience (both direct and indirect) is that crime or acting immorally very often does pay.

Darren] You're right to pick me up on the creationist implication of my argument (not at all what I think). I was with your argument all the way until "In other words, we don't need a rule-set, because the proposition itself, if true, implies the existence of one." I think that's circular reasoning, because the truth or otherwise of a proposition can only be tested by reference to a rule-set.