People should be able to develop their abilities and interests and have access to such goods as friendship, artistry, and nature and a political voice. It's possible to be poor and yet have all this, but in a polarized society, and one where culture and adventure have been thoroughly monetised, it is a lot more difficult.

You can reasonably make the intellectual journey from thinking it's permissible to eat shrimp to thinking it's not permissible, or vice versa, whereas our slavery journey was uni-directional. We are as certain we are not going back to that old kind of slavery as we are that we aren't going back to the geocentric universe.

There's Hobbes, who understood in the 1640s that the sovereign is not an appointee of God, or even a figure of superior virtue and wisdom, but just a functional device whose role is to keep people from hurting and killing each other.

For seventeenth-century astronomers, the Epicurean doctrine of multiple worlds separated by void space was seen to fit with the new Copernican system in which every star was a sun, and the universe was a vast place with no centre.

In Western Europe and North America some things are better than they were - at least relative to their moral nadirs - such as labour legislation, the opening of the professions to women, intolerance for domestic violence, but so much is still morally unacceptable - the weapons trade, cruel and unusual punishment, economic parasitism.

We call 'Slavery is wrong' a moral truth because there is a specific history of theoretical investigation of a particular kind of slavery. We discussed it for centuries in metaphysical, economic, biological, and philosophical terms; we listened to all the arguments pro and con, we read all the testimonies of slaves and witnesses, and we decided. Though this 'we" is not everybody on earth, or even most people, who've never thought about slavery much.

We use and need to use both systems in complex political societies, and we oscillate in our commitments, because both oligarchy and rule by the will of masses have their bad points, as the ancient philosophers all knew.