The theory of moral perception presented in Chapter 8 presupposes moral realism. Moral realism is controversial. Moral rationalists have defended it by drawing an analogy between mathematical knowledge and moral knowledge. It is commonly thought that mathematical facts, such as theorems, play essential roles in scientific explanations and that this explanatory power supports realism regarding mathematical objects. Do purported moral facts, such as true moral principles, have comparable explanatory power? This chapter explores several kinds of explanatory roles apparently played by both moral facts and mathematical facts. In comparing these roles, the chapter shows how moral realism is supportable in ways that draw on an analogy between the explanatory roles of moral facts and counterpart roles played by both certain mathematical facts and certain facts in the physical realm. The case made for moral realism is compatible with the view that moral properties are “naturalizable” but does not require this.