This article is my latest attempt to come up with a minimal version of my evolutionary theory of fairness, previously summarized in my book Natural Justice. The naturalism that I espouse is currently unpopular, but Figure 1 shows that the scientific tradition in moral philosophy nevertheless has a long and distinguished history. John Mackie's Inventing Right and Wrong is the most eloquent expression of the case for naturalism in modern times. Mackie's demolition of the claims made for a priori reasoning in moral philosophy seem unanswerable to me.