From the nature of reality uncovered by science we derive our biological facts. These in turn determine the human facts -- the facts about us, our psychology, and our morality. All cultures, and almost everyone in them, endorse most of the same core moral principles as binding on everyone, and these core moral principles have significant consequences for humans' biological fitness -- for our survival and reproduction. In other words, core morality is adaptive. But the biological facts can't guarantee that morality is right, true or correct. No morality is right, correct or true, and that's nihilism.
The biological facts do guarantee that a certain morality is right, true and correct.
The biological facts say that the goal of living organisms is life. Our biology requires us to exert effort and reason in order to live. Choices and actions are "good" if they support life. A proper morality is one that guides us toward life, by helping us make the right choices.
It's not morality that's "adaptive," it's the underlying philosophy. A proper morality cannot be dictated; it must be chosen.
To adopt nihilism means rejecting not just morality, but reality and consciousness as well. To even have this discussion, one could not be truly nihilistic.
answered Jan 09 '12 at 22:31