Sortal is a concept that has been used by some philosophers in discussing issues of identity, persistence and change. The simplest property of a sortal is that it can be counted, i.e. can take numbers as modifiers. For example, "pea" is a sortal in the sentence "I want two peas", whereas "water" is not a sortal in the sentence "I want water". Countability is not the only criterion. Thus "red thing" in the sentence "There are two red things on the shelf" is not treated as a sortal by some philosophers who use the term. There is disagreement about the exact definition of the term as well as whether it is applied to linguistic things (like predicates or words), abstract entities (like properties), or psychological entities (such as concepts).
According to the Stanford Encyclopedia of Philosophy, the sortal/nonsortal distinction can be characterized in at least six different ways. It is said that a sortal:gives a criterion for counting the items of that kind
gives a criterion of identity and non-identity among items of that kind
gives a criterion for the continued existence of an item of that kind
answers the question "what is it?" for things of that kind
specifies the essence of things of that kind
does not apply to parts of things of that kind
While some philosophers have argued that the notion of a sortal is similar to that of the idea of a "secondary substance" in Aristotle, the first actual use of the term 'sortal' did not appear until John Locke in his 1690 Essay Concerning Human Understanding:
But it being evident, that things are ranked under Names into sorts or Species…the Essence of each Genus, or Sort, comes to be nothing but that abstract Idea, which the General, or Sortal (if I may have leave so to call it, from Sort, as I do General from Genus) Name stands for. And this we shall find to be that, which the word Essence imports, in its most familiar use.
Gottlob Frege is also named as an antecedent to the present debate over sortals. Frege pointed out that in counting things, we need to know what kind of thing it is that we are counting; that is, there needs to be a "criterion of identity."
Sortals make a return with the work of P. F. Strawson, W.V.O. 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."