How did views of slavery change over time and why?

[rufuspollock]

Slavery was supported by most static human societies from the earliest historical times through to ~ mid 17th century.

How did these views develop over time and why did they change and why?

Additional questions

• who in human history questioned slavery?

Notes

• Against the grain
• https://www.bbc.co.uk/ethics/slavery/ethics/philosophers_1.shtml