Even the poorest of societies can avoid holding slaves or placing women or men in legally inferior roles.
To avoid having slaves, the poorest society could decide to kill all war captives, and to let starve to death all people unable to pay their debts. Yes, this would avoid legal discrimination. Is it therefore a morally preferable solution?
Since when has the institution of slavery been a charitable one? Historically, slave-owners have payed immense costs, directly and indirectly, for the privilege of owning slaves, and done so knowingly and willingly. It is human nature to derive pleasure from holding power over others.
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