Quine, Peter Geach, and David Wiggins. Strawson holds that sortals are universals, Quine thinks they are predicates, and Wiggins sees them as concepts. Geach did not use the exact term "sortal"; however, his idea of the "substantival expression" is identical or nearly so to that of "sortal". According to him, identity is relative in a sortal concept, which he described as one that answers the question "Same what?". See also Taxonomy (general) References Further reading E. J. Lowe. More Kinds of Being: A Further Study of Individuation, Identity, and the Logic of Sortal Terms (Wiley-Blackwell, 2009) John Locke