Aristotle's theory of natural slavery, set out in the Politics and in the Nicomachean Ethics is a useful starting point for thinking about slavery in the history of political thought, and for introducing and developing some of the key themes of this book. The idea of conceiving of the slave as an animate tool raises a whole set of questions about the supposed subhumanity of the slave and how that status is understood in the history and politics of slavery. I am particularly interested in the ‘incompleteness’ of the slave, and the ways in which his or her soul was understood to be lacking in spirit, in the constituent elements required to build a free citizen. Aristotle is an important place to start because his arguments bring together political slavery and what Mary Nyquist calls psycho-ethical slavery, and because his explicitly political approach to slavery draws attention to the question of how we should theorise the relationship between slavery as metaphor and slavery as lived experience. Did his theory of natural slavery have anything to say about the lives of actually existing slaves in ancient Greece?

Nyquist discusses the differences between figurative, political slavery and chattel slavery, where political slavery is about the threat to the democratic polis and not about the condition of chattel slaves. Her analysis of the ‘polyvalent metaphor of slavery’ (Nyquist 2013, 5) draws attention to the ‘entangled interrelations’ (Nyquist 2013, 2) between political servitude and chattel slavery, and these entanglements are particularly gnarly in Aristotle's theory. Within the polis, political slavery is represented as the illegitimate domination of free, male citizens who expected to participate as equals in the political process, exercising their freedom as political agents, none of them ruling over others. Political slavery comes about when ‘a leader fails to protect the citizenry's freedom, instead attempting to become its master’ (Nyquist 2013, 22). As Nyquist points out, the injustice of this political enslavement lies ‘in the attempt to enslave those who patently ought not to be enslaved’ (Nyquist 2013, 23). Participants met in the political arena as equals, but they were masters within their own households. Aristotle's opposition to political slavery was not an attack on slavery as wrong in itself.