On Human Nature
If species are the things that evolve at least in large part through the action of natural selection, then both genetic and phenotypic variability are essential to biological species. If all species are variable, then Homo sapiens must be variable. Hence, it is very unlikely that the human species as a biological species can be characterized by a set of invariable traits. It might be the case that at this moment in evolutionary history, all human beings happen to possess a particular set of traits, but if so, this will be in large part an evolutionary accident. As a result, anyone who proposes to base anything, including ethics, on human nature is basing it on historical happenstance.