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This essay clarifies the relations between positive economics and mainstream normative economics. If rationality can be characterized by conditions on preferences and choices and, in addition, people are, to a reasonable degree of approximation rational, then economists can invoke the theory of rationality to explain choices. If people are also, as in positive economics, largely self-interested and reasonably well-informed, then their preferences will also indicate their level of well-being. Modeling both choice and welfare in terms of rational preferences thus unites positive and normative economics. In explaining market outcomes in terms of individual choices, explaining choices by preferences, and taking preferences to indicate well-being, economists who are concerned with welfare defend a normative theory that piggybacks on their positive theory. This traditional mainstream normative theory is problematic, but because of its unity and its connection to positive economics, it is also very powerful.