According to Singer, to the extent that this intuition is not adaptive, we can think of it as “a truth of reason” rather than another value that emerges from competition and selection.
If we apply this to the dualism of practical reason, then what we have is, on the one hand, a response—the axiom of universal benevolence—a response that clearly would not be likely to have been selected by evolution, because to help unrelated strangers—even at some disadvantage to yourself, where there’s a greater benefit to the unrelated strangers—is not a trait that is likely to lead to your improved survival or the improved survival of your offspring. It’s rather going to benefit these unrelated strangers who are therefore more likely to survive and whose offspring are more likely to survive. So that doesn’t seem like it would have been selected for by evolution, which suggests that maybe it is a judgement of our reasoning capacities, in some way; we are seeing something through reason.
Now, if we compare that with the egoistic judgement that I have special reasons to prefer my own interests to those of strangers, it’s more plausible to think that that would have been selected by evolution. Because, after all, that does give you preference for yourself and your offspring, if you love your offspring and care for them.
So if we have these two conflicting judgments, then maybe we can choose which one by saying: just as in the case of adult sibling incest, we debunk the intuition by saying, “Well, that’s just something that evolved in our past and that doesn’t really give us reasons for thinking the same thing today,” maybe we can say that also about the intuition behind egoism, but not about the intuition behind universal benevolence, which therefore gives us a reason (not a conclusive or overriding reason) for thinking that it’s the axiom of universal benevolence that is the one that is most supported by reason.
Nope. The way we reason is a product of natural selection. You don’t get to pick some bits you like and say that these are universal truths.