 Wright (1983) argues that one of Frege’s most important contributions was emphasizing that numbers are objects and that this is intimately connected to Frege’s claim that “natural number” is a sortal. Wright is aware of the difficulties in characterizing sortals, which we will explore below, but remarks that “…whether or not it is ultimately rigorously explicable, the intuitive notion of a sortal concept is clear enough for our immediate purpose” (1983: 4). For further discussion see the entry on Frege.