Some good comments from Gary and Kenny; let me return to those in a moment.
Pinker had another point in The Stuff of Thought (another of his books that I just finished and am still digesting) that seemed relevant to the discussion, about the nature of taboos. A taboo is a cultural norm, but it's not just that a certain action is proscribed, but rather that even directly thinking about something is forbidden. You can violate a taboo just by mentioning it in conversation, or otherwise inviting other people to think thoughts that should not be thought. Taboos are kinda weird, because it's not that the taboo subject is completely unknown to the people who respect the taboo – it's just that they don't want to think about it directly, or behave as if it's a subject for rational contemplation.
Sex used to be a taboo subject in our culture. You could offend someone just by mentioning its existence. Everyone, of course, knew all about sex and certainly engaged in it – it was just not a subject for conversation or contemplation. Over the last fifty years or so that taboo has faded considerably, given what you see on TV, but you will still see some boundaries in what most people will comfortably talk about in public. Religion was also a great taboo that has faded over time; there is more talk now about the "religious marketplace" in which people freely mix and match religious traditions and beliefs. But with religion, too, we become conscious of certain taboos when we see them violated. Recently, the atheists started crossing a taboo line by boldly, publically declaring the non-existence of God. Lots of thinkers declined to believe in divine personage over the centuries, but their opinions tended to be quiet and oblique. It is still largely considered rude to announce one's atheism loudly in a dinner party.
Why do we have taboos, if everyone knows about what we're not talking about? In The Stuff of Thought, Pinker suggests some taboos are related to the terms on which we define our relationships. The anthropologist Alan Fiske categorized all human relationships into three broad categories: Communality (freely sharing among kin and community), Authority (using power to claim resources), and Exchange (trading resources for mutual gain). Each kind of relationship has its own rules and logic, and if you mismatch the rules to the situation, the result is a faux pax. A man who gets up from his mother's Thanksgiving feast (Communality) and offers to pay her $200 for it (Exchange) will certainly offend her and everyone present. Likewise, if a waiter in a restaurant provides good service in hopes of receiving a big tip (Exchange) and instead gets a hearty "thanks!" from the patron (Communality), the waiter will be offended.
Some topics, then, are taboo because they frame the relationship with the wrong set of instinctive relationship-logic. Pinker gives the example of the pre-nuptial contract. Since half of all marriages in our country now end in divorce, you might logically think everyone ought to get a pre-nuptial contract spelling out how a divorce would be handled. Yet most couples strenuously resist even discussing prenuptials, because it engages Exchange mentality ("I'll give you this if you give me that") at precisely the time they want to be emphasizing Communality ("what's mine is yours and what's yours is mine") to ensure a committed marriage. Even thinking the thoughts can bring about the outcome they are trying to avoid in the first place.
I think religious taboos follow a similar sort of logic. People don't want to discuss religion because it undermines the intuitive logic of Communality ("we are all children of God") and Authority ("He alone is the Lord") which defines the nature of our relationships with each other. Whether there actually is a God is not the point – we just want to keep treating each other as if there is a God. And while specific religious traditions have blurred into one another in the multicultural melting pot, the most basic common elements of religion as a definer of our relationships persists: a belief in God (though barely defined), and a vague consensus on a moral reality.
Which brings us up to the current conversation about the genesis of morals. Whenever I pursue moral arguments out to their logical ends (as Singer does), I wind up standing at the precipice of this final taboo: no one wants to admit that our morals are hanging in space with no visible support. Even atheists such as Pinker, who are perfectly comfortable declaring God to be "palpably unreal," are unwilling to say that "there is no moral law." Even after demonstrating that the emotions that support morality and ethics are evolved mechanisms, he will still vehemently insist there really is such a thing as right and wrong. Pinker suggests that maybe morality is the necessary result of logical truth: it always stands to reason that everyone is better off if everyone helps everyone else. Maybe our brains evolved the moral emotions to perceive an objective truth, just like we evolved the mechanism to understand mathematics. These explanations are hardly explanations; to define morality as merely enlightened self-interest is just to say, once again, that we only have self-interest and all we do is look out for ourselves. To say that morality is character of the universe simply begs the question: morality is real because it's real. I have to suspect that Pinker is holding back for reasons other than logic. Perhaps even he can't override the programming in his brain that says right and wrong are real. Or perhaps he just knows it's taboo: to declare morality to be non-rational is social suicide. Nobody trusts the man who has no law.
I'm impressed that you're back up and blogging so soon after the baby! I just posted my first essay in over two months, half of which I'm blaming on Joyce's back. The other half I have no excuse for at all. Sigh...
ANYway, there are a lot of threads here to comment on, but right now let me just hit your main point, and I may come back to the others later.
Descartes made a very strong distinction between the kind of truth you look for in philosophy--the truth of absolute certainly beyond a doubt--and the kind of truth that you have to use as a basis for action, which is more probabalistic, based on the best evidence you have at the moment. Both are important, but don't get them confused. That's the big distinction I see here. I want moral truth on the level of absolute certainty--this is, in fact, the very heart of my own justification for the life of the mystic (shameless plug for another of my own essays, I know, but really, it's all based on the necessity of finding some absolute basis for morality, and the impossibility of doing so through reason).
At the same time, there is the practical level that you have to act on. From all I can tell, right now, well-fed people are better than starving people. That unjustified assumption gives enough basis for making decisions.