 One classic problem for interpretationism is that our dispositions massively underdetermine contents. I believe that (healthy) grass is green. But for some interpretations of  ‘grue’,  ascribing to me the belief that grass is grue will fit my dispositions just as well. As Lewis points out towards the end of “New Work For a Theory of Universals” (1983e), if we are allowed to change the interpretations of my beliefs and desires at the same time, the fit can be made even better. This looks like a problem for interpretationism.