It seems we often see these day very clever (and sometimes less so) people trying to derive moral norms by reasoning forward from basic principles. To take a rather basic example: just about all human societies have had it as a norm that killing another member of society without just cause is wrong. The points of difference are on what constitutes "just cause" and what constitutes "member of society".
One must then decide whether to reconcile these cultural views, or instead proclaim (as some do) that moral norms vary by time and place. I would tend to argue that one simply cannot take moral norms at all seriously and yet argue that they can be fundamentally different depending on when and where you live. In ancient Rome a pater familias was considered to have life and death power over his family (at least in the earliest times -- later Roman civilization increasingly recoiled against this) and thus could (with full moral justification) decide that it was best for the family if one of his children was killed. One may say that this was right or that it was wrong, or that it is a morally indifferent issue (though that certainly seems a hard position to maintain in this case) but one cannot sensibly maintain that it was right then but wrong now -- unless by "right" and "wrong" one simply means "customary" and "not customary" or "beneficial to society" and "not beneficial to society".
If we take it that some things are always wrong and others always right -- that there is some sort of objective moral code which applies to us as a species, or as rational creatures -- the question then becomes how to determine which things are right and wrong, given the differences between various society's moral codes. Many seem to take some general principle as a given, and then reason onwards from there. Thus, in addressing the question of murder, many people who believe they are reasoning from first principles argue that murder is the unjust killing of another being who possesses consciousness and self-awareness, and thus unjustly depriving that being of those faculties. Others argue that it is the ending of any life that is the inherent wrong, and thus advocate vegitarianism.
And yet both of these, in fact, are not working from first principles, but rather are working backwards for certain assumptions -- assumptions that are perhaps so deep-seated that they hardly seem like assumptions. Thus, when someone assumes that personhood (and an inherent right to life) follows from the possession of self-awareness and consciousness, that person implicitly accepts an assumption that killing a human is different in a fundamental way from killing an animal. Consciousness and self-awareness are then seized upon as the most obvious differences between most humans and other animals. Similarly, someone who recoils from all animal killing and advocates vegitarianism implicitly assumes that there is a fundamental difference in kind between animals and plants which makes the killing of the one acceptable but the other not.
Where then do these assumptions come from? Perhaps a true skeptic would say that these are but emotional reactions to the world, unrelated to either reason or reality. But I find this approach to be so unappealing as to seem untrue. Rather, it seems to me that the human person possesses some basic understanding of right and wrong -- which may be obscured to a greater of lesser extent by one's cultural conditioning and personal inclinations and desires. From this (in the absence of some other source that one chooses to trust on such matters, such as revelation) one must take those things which seem most clearly certain and then reason backwards to the general moral principle -- which may itself proceed to resolve some cases on which one is less sure by instinct.
It seems to me that it is in violating this basic principle that Peter Singer's controversial ideas about infanticide clearly go wrong. Singer has identified what he believes to be the characteristics of a human that make killing it wrong, and upon applying those characteristics finds that killing an infant is not murder. Yet, instead of taking it that this means he has chosen the wrong attributes as being the essential characteristics that define the right to life, he assumes that society as a whole has poor instincts as to what deserves the full protections of 'personhood'.
Using the backwards approach, it seems to me instead that one must take our basic revulsion at the idea of killing an infant as a starting point, and work back from that to a definition of what makes murder wrong that explains that killing an adult is wrong and that killing an infant is wrong. From there one can ask: If killing a human that does not yet possess self-knowledge is wrong, then what is it that makes killing wrong? Is it the very identity of being human? This seems a fit to me, since being human is a binary characteristic -- while such things as consciousness and self awareness admit to levels of degree. From there, the question that one asks as you push out into the more ambiguous waters of abortion or euthanasia is: Are we dealing with the killing of a human in this situation? Has there been some sort of change in kind, where we are not longer dealing with a creature of the same sort as another human? Or are we looking at a person who is the same in kind, but at another point in the range of development of function?