> You can't own human beings, because it violates their fundamental rights. This was true back when slavery was "legal".

There's no such thing as fundamental rights. Those were won by sweat and blood.

Up until the enlightenment and humanist eras there was not even the idea that such a thing as rights even exists. Some inkling of rights existed for royalty in the codes of conduct and such things, but they were freedoms given by right of birth and power of ancestor's sword.

For peasants, no such luck. They were property even though they weren't slaves per se.