Plato extends a bold, confident, and surprising empirical challenge. It is implicitly a claim about the psychological — more specifically motivational — economies of human beings, asserting that within each such economy there is a desire to live well. Call this claim ‘psychological eudaimonism’ (‘PE’). Further, the context makes clear that Plato thinks that this desire dominates in those who have it. In other words, the desire to live well can reliably be counted on (when accompanied with correct beliefs about (...) the role of morality or virtue in living well) to move people be virtuous. As we will argue, this general claim appears in not only Plato but Aristotle and the Stoics as well. But it is one we might wonder about, in three ways. First, we might wonder about its warrant. After all, the claim is universal in scope; yet it is about a highly contingent fact about the motivational propensities of individual human organisms, and there is abundant variability in the individual forms human nature takes. What grounds could the ancients have for their confidence that there are no outliers (assuming, as we do, that they do not merely misspeak in framing general claims as universal ones)? Second, we might wonder about its truth. For were it true, it would entail something remarkable about the nature of rationality that we (post-)moderns would be wise to heed. And third, we might wonder about its relationship with normative eudaimonism. By ‘normative eudaimonism’ (‘NE’) we mean the claim that we have conclusive reason to act in ways that conduce to our own eudaimonia. As we will show, the key to these three questions is the first. If we consider what justification the ancients have for their claim, we can see why that claim must be true. Moreover, as we will also show, it must be true because of the nature of practical rationality as the ancients understood it — that is, in terms of normative eudaimonism. We will show this by marshalling unexpected resources: Donald Davidson’s work in understanding how we interpret others and in so doing make sense of them as rational beings. If we couple Davidson’s account of interpretation with the eudaimonist structure of practical rationality essential to these ancient ethical theories, psychological eudaimonism is a consequence. The paper proceeds as follows. In Section I, we lay out the textual basis for ascribing PE to Plato, Aristotle, and the Stoics. In Section II, we introduce Davidson’s account of interpretation. This allows us to appropriate that account in Section III to the particular purposes of normative eudaimonism, to support the claim that we must ascribe the desire to live well to those whom we would see as rational. Finally, in Section IV we consider challenges to this strategy. (shrink)