Slavery is the institution that most challenges any simple narrative of 'ancient progress.' Civilizations we admire for their philosophical sophistication, their democratic governance, and their artistic achievement were simultaneously slave societies in which a substantial fraction of the population had no legal personhood.

The estimated scale is difficult to establish but clearly large. Athens in the classical period may have had 80,000-100,000 slaves — roughly one-third of the total population. Roman Italy at the height of the Republic may have had 2-3 million slaves, again roughly one-third of the population. The Hellenistic kingdoms and Roman Egypt maintained large slave populations.

The mechanisms of enslavement were multiple: warfare (the most important source), piracy, birth to slave mothers, debt bondage (formally abolished in Athens by Solon, but other forms of coerced labor persisted), and long-distance trade from regions outside the slave-holding societies. The Euboean and later Roman slave-trading networks reached into the Black Sea, sub-Saharan Africa, Central Asia, and Britain.

The most uncomfortable aspect of ancient slavery for intellectual historians is the relative silence of ancient philosophy about its injustice. The Stoic argument that external circumstances including enslavement are morally irrelevant was the closest the ancient world came to a philosophical engagement with the institution, and it's an argument that rationalized rather than challenged the status quo. The primary ancient critique of slavery was not moral but economic — Aristotle's defense of 'natural slavery' is explicitly a response to those who argued the institution was merely conventional.