The moves to contractualism and utilitarianism required some extra ingredients besides mortalism, the denial that God is in charge of the world, and the doctrine that physical and psychological pain are the greatest evils.

Oddly, since by now I've written quite a lot on early modern philosophers, I didn't care for the history of philosophy, which I thought dull and obscure, until I got a minor job writing articles for a children's encyclopedia in the history of science and began to make connections between science and philosophy.

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.

Even if the gods did exist, the Epicureans argued, they didn't care about us. Rather, everything comes from nature, and all that really exists are atoms and void, moving and congregating.

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.

There is no single test or formula for producing moral progress anymore than there is for generating scientific truths. It is a process involving theoreticians, fact-gatherers, protestors, martyrs for the cause, authors of first- person narratives who change the way we see and evaluate the distribution of harms and benefits.

Claims like 'Slavery is wrong' are not fully common-sensical, so they must be at least partly theoretical.