Here is the worry. The scientific outlook has taught us that some parts of our subjective experience are products of our biological makeup and have no objective counterpart in the world. The qualitative difference between red and green, the tastiness of fruit and foulness of carrion, the scariness of heights and prettiness of flowers are design features of our common nervous system, and if our species had evolved in a different ecosystem or if we were missing a few genes, our reactions could go the other way. Now, if the distinction between right and wrong is also a product of brain wiring, why should we believe it is any more real than the distinction between red and green? And if it is just a collective hallucination, how could we argue that evils like genocide and slavery are wrong for everyone, rather than just distasteful to us?"