I hope Massimo won't start regretting his generous invitation for me to co-blog with him (hi readers! great to be here!) if I kick things off by immediately and publicly disagreeing with him. He and I have been having a debate on moral philosophy for the last few weeks, and after the twentieth iteration of the same arguments we decided it makes sense to invite you all to weigh in, at the very least because we're tired of the sound of our own voices by now. Massimo asked me to lay out the debate, and then he'll follow up with his own post next week.
So, I agree with Massimo that moral reasoning is possible, given a set of initial axioms. (Axioms are the starting assumptions on which all of your moral judgments are based, like the concept of certain fundamental rights, or tit-for-tat justice, or protecting individual liberty, or maximizing total happiness). Where I disagree with him is over his belief that it is possible to use scientific facts to justify selecting one particular set of initial axioms over another.
The difficulty of deriving facts about how people ought to behave from facts about how the world is was most famously articulated by David Hume in his A Treatise of Human Nature (1739):
"In every system of morality, which I have hitherto met with, I have always remark'd, that the author proceeds for some time in the ordinary ways of reasoning, and establishes the being of a God, or makes observations concerning human affairs; when all of a sudden I am surpriz'd to find, that instead of the usual copulations of propositions, is, and is not, I meet with no proposition that is not connected with an ought, or an ought not. This change is imperceptible; but is however, of the last consequence. For as this ought, or ought not, expresses some new relation or affirmation, 'tis necessary that it shou'd be observ'd and explain'd; and at the same time that a reason should be given; for what seems altogether inconceivable, how this new relation can be a deduction from others, which are entirely different from it."
With that in mind, I can't see any way in which a claim of the kind Massimo is making -- "doing X increases human welfare, therefore X is the moral thing to do" -- could logically hold, unless you're simply defining the word "moral" to mean "that which increases human welfare," in which case the statement is tautologically true. But I'm not sure what we gain by simply inventing a new word for a concept that already exists.
Fortunately, even though I think the blade of Hume's guillotine is inescapably sharp in the philosophical world, I don't think it has the power to sever much in the real world. Because, thanks to some combination of evolutionary biology and social conditioning, I do enjoy being kind, and I do want to reduce other people's suffering -- and I would want to do those things even without a rational justification for why that's "moral." And I believe most people would feel the same way.
Massimo, I believe I've represented our disagreement accurately, but please correct me if I haven't! *thwack* Ball's in your court!