I would argue that under the atheistic system altruism does not exist.
I'd dispute this, actually. There are idols outside the self---Kantianism is all about altruism and self-denial.
Really? So you think that a person, particularly an unbelieving person, can TRULY do something without ANY self-seeking involved whatsoever? Yes, Kant argued that in order to truly do something altruistically a person would have to find absolutely no pleasure in it. In fact, they would essentially have to grudgingly force themselves to act for the benefit of others in order for their action to have moral worth.
By the way, if you believe that man is totally depraved, and enslaved to sin, then what WOULD man's ultimate end of his actions be? They would always be self-seeking, whether directly or in-directly. All man's actions are a means to an end, and the end (for the sinful man) is himself (for he has rebelled against God and attempted to usurp his authority).
By the way, I don't think Christian ethics is altruistic either. We are in it to enjoy God, remember? Why did Jesus endure the cross, according to Hebrews? For the joy that was set before Him.
So are you saying that we ONLY love God because we get something out of it? Loving God and finding satisfaction in that relationship with God is VERY different then loving God IN ORDER TO find satisfaction. Consider what the Apostle John says:
1 John 4:15-19 (NASB)
15 Whoever confesses that Jesus is the Son of God, God abides in him, and he in God.
16 We have come to know and have believed the love which God has for us. God is love, and the one who abides in love abides in God, and God abides in him.
17 By this, love is perfected with us, so that we may have confidence in the day of judgment; because as He is, so also are we in this world.
18 There is no fear in love; but perfect love casts out fear, because fear involves punishment, and the one who fears is not perfected in love.
19 We love, because He first loved us.
I would honestly say that based on this, Christians are indeed altruistic. The ultimate goal of our actions is the glorification of God, not of ourselves. We do not glorfiy God IN ORDER to please ourselves, but we do indeed find pleasure in glorifying God. It is a question of causality. We do not love God in order for him to love us (this statement would be made by someone who is self-seeking). Instead, we love God BECAUSE he loved us first. We do indeed find pleasure in loving God, but that is not WHY we love him. For this reason Christians, in their glorification of God, are NOT self-seeking.
Why should human beings be moral?
Why not? It's pretty blasted obvious that they should.
Obvious to who? It's only obvious because you believe in God. I am fairly certain that the Nazis would laugh at your statement that it is obvious that humans should be moral (unless of course you were to define morality in a way that they agreed with).
So in the end, the hypothetical atheist is using the term 'moral' in both of his premises. Well, he first needs to explain what he means by moral
But Eric, not having a convincing answer is very different from being inconsistent or self-contradictory. Our hypothetical atheist is challenging you to find a contradiction, not an unanswerable question.
I never mentioned having a convincing answer, and I don't think it matters just how 'convinced' the atheist is (no matter how convinced he is that he is right, he is still wrong). I said that ANY attempt on his part to come up with an objective moral system will be fallacious. It will be illogical and be reduced to absurdity. In fact, it would be IMPOSSIBLE for him to come up with an objective morality. If he tries to appeal to the majority, he needs to pick a society as his final authority. But societies change over time. Furthermore, he has deal with the issue that the majority is not always right. If he wishes to appeal to nature, he has to bridge the gap between IS and OUGHT. Ultimately his system will lead to moral relativism, and he will have to conclude that there is no such thing as morality (it is just an arbitrary human construct that changes between time, culture, and location).
Generally, atheists AVOID having to conclude in moral relativism by doing a number of things:
1) They exchange one relative/ambiguous term for another (happiness, hurt, harm, bad, good, usefulness, practical, common sense)
2) They divert the argument, change direction in order to avoid the path that their worldview leads them down
3) They borrow from a worldview that does not lead to moral relativism (Christianity)
But the one thing they cannot do is remain consistent with their atheistic worldview and end up with a system of objective morality. If they did do this, it would prove that God is unnecessary for there to be morality (because obviously they found an objective system of morality without any reference to God).