Argument Three. A definition must provide the genus of the thing defined, and the specific difference by which the species defined is distinguished from every other species belonging to that kind. Thus only a species can be defined. But a proper name be defined, for individuals are not species. They have no specific difference, and can only be distinguished by the proper name itself. Thus the meaning of a proper name is the individual it names.
Suggested by Reid again. Of the Intellectual Powers, pp 219-20. See also Argument 1, argument 2.