Case 1: Your ancestors evolved (genetically or mimetically) a belief that a local forest was full of tigers. This belief has kept your ancestors out of the forest where they would have been eaten by tigers, and that is why the belief evolved.
Case 2: Your ancestors evolved a belief that a local forest was full of tigers. This belief has kept your anceestors out of the forest where they would have been killed by snakes and that is why the belief evolved—a belief in tigers, let us suppose, is better at keeping people away thant a belief in snakes. Coincidentally, the forest is full of tigers, but these tigers are shy—they silently run away when people approach, and no one has ever had an encounter with one.
If Case 1 is a case of knowledge, Case 2 is a Gettier case, and hence not a case of knowledge that the forest is full of tigers. And if Case 1 is not a case of knowledge, Case 2 isn't either. So, either way, Case 2 is not a case of knowledge.
Case 3: Your ancestors evolved moral beliefs. These beliefs kept social cooperation going and that's why they evolved. The beliefs are by and large true.
In Cases 2 and 3, unlike in Case 1, there is no relevant connection betweeen why your ancestors evolved the belief and the belief's truth. Case 3 is relevantly like Case 2: it is not a case of knowledge but at most of Gettiered justified true belief.
A story like Case 3 is the only plausible account of the genesis of moral belief available to the non-Aristotelian naturalist. Since we do know moral truths, mon-Aristotelian naturalism is false. Can the Aristotelian do better? Maybe. Moral truth is grounded in natural human inclinations, she can say, and the same inclinations often give rise, noncoincidentally, to correct moral beliefs. I wonder if the only way for a naturalist to be a realist about morality is for her to be an Aristotelian.
