As an extreme condition, slavery provided the free with a metaphor and a yardstick for a variety of relationships. Aristotle had mapped out political, social and familial relations in terms of dominance and subordination, drawing a series of analogies and distinctions between the different relationships, and others followed suit. Cicero's version in De republica 3.37 (=Augustine Contra Julianum, 4.12.61) is particularly interesting:

Cicero identifies three spheres of domination: the state, the family and the individual. The slave-master relation takes its own place within these, as a sub-category of familial relations, but slavery also provides a principle of differentiation: you can't dominate a citizen or a child as you would a slave; the body is to be guided, but the desires are to be compelled and chastened like slaves. Failure to observe these distinctions leads to slavish dispositions in the relevant sphere.