 Take a predicate like ‘green’. The totality of facts about our previous use and dispositions to use ‘green’ are consistent with it referring to the property green but also with it referring to the property grue. So, what might ground the claim that the property green is somehow privileged as the referent of ‘green’? Lewis can be taken to advocate a form of “interpretationism” according to which semantic facts are constitutively determined by the best theory of the data (J. R. G. Williams 2007). Among the a priori constitutive constraints governing what counts as the best theory is a principle requiring that the referents assigned to expressions be the most natural of those consistent with the data. Since green is more natural than grue, it is more “eligible” than grue to be assigned to ‘green’ as its referent. Likewise for adding and quadding. In this way, the indeterminacy left open by facts about use is fended off, Lewis thinks.