 The descriptivist account, however, faces two particular problems with regard to the historic evolution of scientific theories. First, if descriptions of theoretical entities are constitutive of the meaning of corresponding unary predicates, one must wonder what the common core of understanding is that adherents of successive theories share and whether there is such a core at all. Were Rutherford and Bohr talking about the same type of entities when using the expression ‘electron’? Issues of incommensurability arise with the descriptivist picture (Psillos 1999: 280). A second problem arises when elements of the description of an entity in a given theory T are judged wrong from the viewpoint of a successor theory \(T'\). Then, on a strict reading of the descriptivist account, the corresponding theoretical term failed to refer in T. For if there is nothing that satisfies a description, the corresponding expression has no referent. This is a simple consequence of the theory of description by Russell in his famous “On Denoting” (1905). Hence, an account of weighting descriptions is needed in order to circumvent such failures of reference.