The goal of this article is to explore some fundamental assumptions underlying subjective measures of well-being, as compared to more traditional economic measures. Its main thesis is that psychologists and economists have sharply different philosophical commitments, a fact that is seldom made explicit. Although it is perfectly reasonable for social and behavioral scientists to be wary of spending too much time thinking about the philosophical foundations of their enterprise, there are moments when it is eminently useful to do so. In this case, this article maintains, there is good reason to attend to these foundations, since they are directly relevant to the assessment of the various measures. A better grasp of fundamental commitments, this article argues, goes a long way toward explaining why psychologists' and economists' efforts to measure welfare or well-being are so different, and why there is relatively little fruitful communication and collaboration across fields.