David Hume had this idea that if we see an action and it seems good to us, then there's a kind of evolutionary selection to make sure that actions that seem good to us are also morally good. Adam Smith elaborated this in his system of propriety when he talks about the four different origins of moral sentiments. He thought that over time, human beings, both individually and collectively, were capable of honing of perfecting this sense of propriety. And so that much of our behavior would actually be governed by the reactions of others which we had internalized to such an extent.