There is a long-standing and persistent problem concerning interpersonal comparisons of well-being. In its strongest versions, the allegation is that it is literally not possible to make useful comparisons of the happiness or well-being of different persons (or, even the same person at different times). An interpersonal comparison of value would be “useful” if we could know, for example, that Smith is happier than Jones, or that, taken together, the Smiths are happier than the Joneses. This kind of comparison crucially involves quantitative comparison in terms of better than, worse than, or equally good, or the sort of aggregation we employ when we add Mr. Smith’s well-being to that of Mrs. Smith and compare the sum to that of Mr. and Mrs. Jones.
On some interpretations, the problem is fundamentally epistemological in nature—it arises because facts about how other people are doing are closed off to us and unknowable in an important way. But on another interpretation, the problem arises because meaningful quantitative comparisons can be made only if the comparison is made relative to a single degree-admitting feature, such as height, or mass, temperature, or some such thing. This is why it does not make sense to claim that 2 grams is greater than 1 meter, or to add these quantities together—the numbers are being used to represent inherently different degree-admitting features of objects. 2 grams plus 1 meter does not equal 3 of anything.
Another instance of this sort of problem came up a couple of years ago, when Oscar Pistorius, a double amputee track-and-field athlete who runs using prosthetic carbon fiber blades in lieu of feet, was trying to get permission to run in the Beijing Olympics. He was not allowed to compete because, it was said, his blades confer an unfair advantage over people who run the old-fashioned way. The root of the problem is that it is unclear whether these modes of transportation are comparable, in the sense that they are the same thing, and so it is unclear whether it is possible to (directly) compare the results of the two modes of sprinting. If they are not the same thing then the blades should not be allowed in footraces for the same reason (though to a lesser extent) that bicycles are not allowed in footraces—running and cycling are too different, and so it is not possible to compare the results of those two modes of transportation. And so the question as to whether the blades confer an unfair competitive advantage is the question as to whether what Pistorius is doing is similar enough to what the other racers are doing to license a comparison.
This is relevant to the problem of interpersonal comparisons because, given the inherent and essential perspective-dependence of a person’s well-being, Smith’s well-being is not the same thing as Jones’s well being, and Mr. Smith’s well-being is not the same thing as Mrs. Smith’s well-being, in exactly the same way that being good for making cheesecake and being good for making coffee are not the same thing. And in a way that is similar to the way that running and running using carbon fiber prosthetic blades are not the same thing (if they’re not). And this is supposed to explain why attempting to compare or aggregate the well-being of different people is impossible: the alleged comparisons inherently involve what can only be described as different features, in violation of a clear necessary condition for the making of a legitimate, coherent comparison.
But it seems to me that this is not right. It is possible to compare “goodnesses” even if the goodnesses that are objects of comparison are not precisely the same kind of goodness. It makes sense to claim that I am a better guitar player than a drummer, even though good at guitar and good at the drums are not the same way of being good. It is possible to make comparisons with respect to different dimensions as long as the dimensions are relevantly similar, and it is impossible to make comparisons when the dimensions are insufficiently similar. So, for example, being good at guitar and being good at the drums are similar enough to license comparison, but being good at guitar and being one meter long are not. While not identical, being good for Smith and being good for Jones are much more similar than are being good at guitar and being good at the drums. And they are close enough that it makes sense to compare their instances with respect to goodness.