 Whatever metric they use, all instrumental theorists will have to address longstanding questions about the commensurability and interpersonal comparison of interests. Since instrumental theorists work with overall evaluations of how well off individuals would be were certain rights ascribed, they must explain how distinct categories of interests (e.g., health, income, opportunities for self-expression, social recognition) trade off against one another. They must also explain why they believe that these interests are similar enough across persons that it makes sense to use the same scales of measurement for different persons (Griffin 1989).