At the beginning of book three of his Treatise on Human Nature, David Hume argues that justice is something we invent. In a word, justice is unnatural. It isn’t something we just see in the world, since we only ever see what is, and nothing in what we see tells us how things ought to be. Neither does justice come from some inner, natural feeling, if by “natural” we mean the hard-wired, immediate pleasures and pains that we can’t help but have. No; to have any sense of justice, we have to be taught to have it.

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