According to the standard interpretation of Lewis’s theory of predicate meaning (the U&N theory), the naturalness of meaning candidates should be stated metaphysically - as a length of definition in terms of fundamental properties. Recently, Weatherson has criticized the U&N theory and argued that the criterion of naturalness should be stated epistemologically - as the amount of evidence needed to form a belief. Despite the criticism, his attitude towards the U&N theory is quite relaxed. According to Weatherson, the U&N theory can be used as a good heuristic for delivering the correct verdicts when doing applied semantics, i.e., when we try to determine the best meaning candidate for a particular predicate. In this paper, I try to show that the “good heuristic strategy” is of no use because A) there is no guarantee that the epistemological and the metaphysical criteria of naturalness deliver the same verdicts and B) even if they deliver the same verdicts, the difference in their theoretical backgrounds may affect arguments which rely on the verdicts. The difference will be shown by drawing on the example of Theodore Sider and his use of the U&N theory.