From what can 'ought' be derived. The most compelling answer is this: ethics must be somehow based on an appreciation of human nature - on a sense of what a human being is or might be, and on what a human being might want to have or want to be. If that is naturalism, then naturalism is no fallacy. No one could seriously deny that ethics is responsive to such facts about human nature. We may just disagree about where to look for the most compelling facts about human nature -n novels, in religious texts, in psychological experiments, in biological or anthropological investigations. The fallacy is not naturalism but, rather, any simple-minded attempt to rush from facts to values. In other words, the fallacy is greedy reductionism of values to facts, rather than reductionism considered more circumspectly, as the attempt to unify our world-view so that out ethical principles don't clash irrationally with the way the world is.