 For example, co-referential names, such as "Samuel Clemens" and "Mark Twain", cause problems for a directly referential view because it is possible for someone to hear "Mark Twain is Samuel Clemens" and be surprised – thus, their cognitive content seems different.Despite the differences between the views of Frege and Russell, they are generally lumped together as descriptivists about proper names.Such descriptivism was criticized in Saul Kripke's "Naming and Necessity".