I suspect that there's very little in ethical debate that is more important than sorting out where we are making decisions about matters of fact and where we are making decisions about matters of value. Not making this distinction, and not carefully analysing whether our disagreements fall into one category or the other, leads to us talking past one another, thinking ill of one another and also just being muddle-headed.
A question of fact is a question about what is (or is not). An example of a question of fact would be 'there is such a thing as ice-cream'. This particular fact is trivial, and easy to prove, but not all questions of fact have those characteristics. The statement 'there is such a person as God' is certainly not trivial, and is not readily proved to most people's satisfaction, but it is still a question about what is - a question of fact.
A question of value is a question about what something is worth. An example might be 'ice-cream is tasty'. Again, this value statement is trivial. But the statement 'God deserves to be worshipped' is not, and neither is the statement 'murdering human beings is wrong'. These are significant statements, but they are not statements about what is. They are to do with value.
It would be easy to suppose that this distinction is identical to the distinction between what is objective and what is subjective: facts are things that are absolutely true (or false), whilst values are essentially opinion. This may well be true on a naturalistic worldview; it is not true on a Christian worldview, where God gives values which are, from a human standpoint at least, objective and universal. In a naturalistic worldview, you can be wrong about facts, but not really about values; the same does not ring true for the Christian (although obviously some value judgements really are just subjective, like my ice-cream example). This is interesting, because it means that interpreting value statements will ultimately boil down to a fact question: does God exist? But all this by the by...
Getting the fact and value distinction wrong has tainted the abortion debate in the US, I think, and I want us to get it clear on this side of the pond. In this debate, you have two sides: the 'pro-life' people, who think abortion is wrong, and the 'pro-choice' people, who think people should be able to choose. Just so you know, I'm firmly with the pro-lifers. Pro-life people often think that pro-choice people are making a value judgement: perhaps 'a woman's right to choose is more important than an infant's life', or more starkly 'it is not wrong to murder a baby'. Of course nothing could be further from the truth, and we need to drop that sort of rhetoric. The disagreement is one of fact, not value. Nobody thinks it's okay to kill a baby. Some people think that the foetus is not a baby - and that is a question of fact, not value. So the argument needs to proceed on factual questions: is this a baby, or not? Is this human or not?
This is important because often we agree on the value judgement: a human being must not be killed simply because that being's existence is inconvenient or even tragic for another human being. Let us take that as settled. (Though I cannot see why a naturalist should think so, I am grateful for the common grace that gives most people this intuition). Now we can have a sensible discussion about what a human being is, apart from emotive value-language. This won't make the discussion easy - after all, many religious pro-lifers will take their position from revelation to some extent. But I think it is a discussion that can be had.
I also appeal to pro-choice people to understand what is going on here. We disagree with you about facts. That means that we can't adopt a "live and let live" approach (to coin a phrase in a horribly ironic manner) to this issue. We really think that people are being killed, legally, at the point of their greatest defencelessness. If you thought that was the case, wouldn't you do anything in your power to stop it?