You're sniffing around the classic refutation of goodness having its grounding in a god, called the Ethyphro Dilemma. In Plato's Ethyphro dialog Socrates asks (I'm paraphrasing to update the argument) whether what is good is only good because God says it is or whether God merely points to what is good because it's good.
If the first (what your opponent is arguing), what is good is rendered arbitrary. That which is good is only good insofar as God says it is. That means that God could declare that the rape of 3 year old children is good and it would be good simply by virtue of his having said so.
If the second, then what is good is already good; God merely describes it for us. But if that's true, and if it is true that goodness requires an objective grounding like the theist wants to think, then there must exist a grounding for goodness over and above and apart from God. And then question is, who or what is that supposed to be?
RE: "(since god can't order evil things, right?)"
That's not the case, Dave, if you believe the Bible: Isa 45:7 - God says "I create evil."
For God to be or do evil, there would have to be an independent overarching concept of The Good. But of course that is impossible. If God does exist, then He is the source of all, including any standards. And of course, He can also be the exception to any standards He propounds.
I'm playing the Devil's advocate here... or... well, God's (which is worse)... but "evil" is replaced with "destruction" or other words in different translations. To me it only makes sense that God creates evil since he created both Satan and Hell, but yeah... Christians are going to argue that you're reading the wrong translation. Convenient, right?
Cara - tell them to translate this:
Isaiah 45:7 - "I form the light and create darkness: I make peace and create evil: I the Lord do all these things."
Yes, that's the verse I'm thinking of. And I think I did send it to my mom at some point. Like I said, other translations replace "evil" with "destruction". It still makes sense in the context of this verse. I suppose either bad though. If he's responsible for destruction, wtf?
Plato's argument rests on his belief in a world of Ideas which is actually more real than the physical world. The physical world is simply an imperfect reflection (or shadow) of the perfect world of Ideas, which is eternal and unchanging. Now, Plato's "Ideas" are different from the way the term is used in everyday life ("I have an idea how to save money on our electric bill") for his ideas aren't notions, they are objects in a world more real than our own. Objects because they are truly objective—real and eternal and perfect. This raises the ontological question: does such a universe of Ideas actually exist, as well as the epistemological problem of how we would ever know if it did.
To me, Plato's world of Ideas, as attractive as it may be, is not much different from a religion, for it must be believed on faith apart from evidence.
It's unclear to me how that would undermine the force of the Ethyphro, or even if indeed you were saying that it does. There's no question that many if not most of Plato's ideas were wrong either because they rested on his belief in a world of ideas that was more real than the physical world or regardless of this. But the Ethyphro still presents the horns of a dilemma, neither one of which the theist wants to grab a hold of because of the implications for their view of morality.
Oh, I certainly accept the dilemma as a dilemma. I myself am very influenced by the later Wittgenstein such that, when you have a philosophical dilemma like that, it's probably a problem in the asking of the question or or in some hidden, unstated, or subconscious presuppositions.
The dilemma disappears with the absence of God.
There may be something semi-universal in the psychology of man telling us (probably based on empathy) what is right and wrong, such that if someone disagrees with a certain particular ethical judgment, there must be something wrong with them. Normal people may disagree over the circumstances when it is right or wrong to steal or tell a lie, but if someone were to maintain that it's okay to put out a cigarette in a baby's eye, most people in just about any culture would find the person holding that view to be a highly defective human being: psychotic or just plain crazy.
At the same time, such judgments may be culture-bound. One culture might see beheading as a proper punishment for murder whereas another culture might find such an extreme measure horrific.
But all that just goes to confirm the idea that, like all politics, all ethics are local.
oh Nelson... if only you were around when I had to debate this in high school!