Aristotle's claim that natural slaves do not possess autonomous rationality (Pol. 1.5, 1254b20-23) cannot plausibly be interpreted in an unrestricted sense, since this would conflict with what Aristotle knew about non-Greek societies. Aristotle's argument requires only a lack of autonomous practical rationality. An impairment of the capacity for integrated practical deliberation, resulting from an environmentally induced excess or deficiency in thumos (Pol. 7.7, 1327b18-31), would be sufficient to make natural slaves incapable of eudaimonia without being obtrusively implausible relative to what (...) Aristotle is likely to have believed about non-Greeks. Since Aristotle seems to have believed that the existence of people who can be enslaved without injustice is a hypothetical necessity, if those capable oí eudaimonia are to achieve it, the existence of natural slaves has implications for our understanding of Aristotle's natural teleology. (shrink)