In this paper I argue that there is a strong fit between the structure of Aristotle’s inquiry into natural slavery in the Politics and the scientific method described in Posterior Analytics II.1–10. This conclusion may seem difficult to sustain in light of the fact that Aristotle’s definitional inquiry into the natural slave (Pol. I.4) precedes his determination of the existence of natural slaves (Pol. I.5), for he repeatedly asserts in the Posterior Analytics that existence is properly investigated before essence. But this problem can be resolved by noting, first, that different kinds of definitions play different roles in scientific inquiry (An. Post. II.10), and, second, that Aristotle continues to refine his definition of the natural slave into Pol. I.5. On the basis of a close textual analysis, I confirm that Aristotle’s treatment of the natural slave in the Politics exhibits the very sort of existential/definitional progression characteristic of the Posterior Analytics II.1–10 method. I conclude by stressing that this thesis coheres with the importance of definitions in practical as well as theoretical disciplines and is not incompatible with the autonomy of the sciences or the remarks about precision in NE I.3.