So ethics, obviously a difficult subject which Prof McGinn approached in a quite logical straightforward manner. I totally recommend you listen to the audio recordings if you can his style is quite approachable and easily understood. The first question (and one that I agree with Prof McGinn on) is about ethics being knowable. That is, are ethical truths the same as truth as we discussed previously? In case you missed that part of the series and don't want to read it, I'll sum it up... Truth is objective based in reality. You can't think something into falsehood. Snow is white no amount of wanting it to be different will change that truth. Now if (and I hold that it is) ethics is knowable truth then it is also not subjective. Also, it must also be a priori as was discussed previously. Again, I'll sum that idea up; a priori is NOT necessarily something that is known from birth, rather something that can be known without experience. The prime example is mathematics, one can know that 1+2=3 but no one can point to a one, two, three, plus or equals (not the symbols, but the objects, which don't exist as we think of existence). So one can know that murder is wrong without being able to see wrong or right in and of themselves. So, knowledge of ethics definitely falls under the realm of a priori knowledge.
Well, there are many that dispute that claim. The primary disagreement is that moral or ethical claims are merely emotive statements, in fact emotivists say they're less than that. To an emotivist, saying, "murder is wrong," is the equivalent of saying "murder, boo." Emotivism is just one of the many attempts to escape the reality of moral truth. The most important thing to take from this lecture is about how non-cognitivists (those that believe morality is not knowable truth) are guaranteed to come away with moral relativism. It's obvious, if saying, "rape is wrong" is equivalent to "boo, hiss" then what's the point of any moral statements? They're all worthless.
Here's where I have some disagreement with Prof McGinn... After talking about how he believes that moral truth is NOT subjective, that it's completely objective and just as trustworthy as mathematical truths. So far, I can agree with him. Then, he starts into a critique of divine command theory. Let me first say, I'm not a fan of the divine command theory. I've read the Euthyphro dialogue (granted, it was a few years ago in college) but I do remember enough to know that Prof McGinn seems to make a mistake, like the one he made when he retold the cave myth from the Republic. He says that Socrates meets Euthyphro while walking around Athens, which is not true to the dialogue. In the dialogue, Socrates meets Euthyphro on the porch of King Archon (steps of the courts) because he's on trial. They strike up a conversation, and the so called, "Euthyphro dilema." I've written about this before, Is something good because God commands it, or is something good commanded by God because it is good? Euthyphro doesn't have a good answer, and as I've mentioned before, I feel that's mainly because of a misunderstanding of the nature of God. The ancient Greek gods were very anthropomorphic and fallible. God as He actually is, isn't fallible as a man, he's immutable, perfection, omnipotence, omniscience, among other characteristics, all of which are the furthest any being can be from humanity. God is not the foundation of morality, God is the definition of morality. Murder isn't wrong because God says it's wrong (per se) rather because it's against the very nature of God and morality.