Aquinas may seem profligate in defining ‘happiness’ (beatitudo). He says, “by the name ‘happiness’ is understood the ultimate perfection of a rational or of an intellectual nature” (ST Ia q.62 a.1 co.). He also says, “‘happiness’ names the attainment of the ultimate end” (ST IaIIae q.2 pro.). He further says the following “definition of happiness” is “good and adequate”: “Happy is the one who has all that he desires” (ST IaIIae q.5 a.8 ad 3). So which expresses what happiness really (...) is? Which gives us the quid est of happiness? In this essay, I argue that his quid est definition of happiness is put in terms of “the attainment of the ultimate end.” I further argue that, once that definition is properly understood, it becomes clear that Aquinas thinks happiness just is intimately knowing and enjoying God. I close by focusing on one downstream interpretive effect that this interpretation could plausibly have; it may influence our understanding of the relationship between virtue and happiness in Aquinas. (shrink)