A sortal is something that takes numerical modifiers. There is disagreement about the exact definition of the term as well as whether the term is applied to linguistic things (like predicates or words), abstract entities (like properties), or psychological entities, (such as concepts).

Differing perspectives

According to the Stanford Encyclopedia of Philosophy

Stanford Encyclopedia of Philosophy

The Stanford Encyclopedia of Philosophy is a freely-accessible online encyclopedia of philosophy maintained by Stanford University. Each entry is written and maintained by an expert in the field, including professors from over 65 academic institutions worldwide...

, 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

History

While some philosophers have argued that the notion of a sortal is similar to that of the idea of a "secondary substance" in Aristotle

Aristotle

Aristotle was a Greek philosopher and polymath, a student of Plato and teacher of Alexander the Great. His writings cover many subjects, including physics, metaphysics, poetry, theater, music, logic, rhetoric, linguistics, politics, government, ethics, biology, and zoology...

, the first actual use of the term 'sortal' did not appear until John Locke

John Locke

John Locke FRS , widely known as the Father of Liberalism, was an English philosopher and physician regarded as one of the most influential of Enlightenment thinkers. Considered one of the first of the British empiricists, following the tradition of Francis Bacon, he is equally important to social...

 in his 1690 Essay Concerning Human Understanding:
Gottlob Frege

Gottlob Frege

Friedrich Ludwig Gottlob Frege was a German mathematician, logician and philosopher. He is considered to be one of the founders of modern logic, and made major contributions to the foundations of mathematics. He is generally considered to be the father of analytic philosophy, for his writings on...

 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."

Contemporary discussion

Sortals make a return with the work of P. F. Strawson

P. F. Strawson

Sir Peter Frederick Strawson FBA was an English philosopher. He was the Waynflete Professor of Metaphysical Philosophy at the University of Oxford from 1968 to 1987. Before that he was appointed as a college lecturer at University College, Oxford in 1947 and became a tutorial fellow the...

, W.V.O. Quine, Peter Geach

Peter Geach

Peter Thomas Geach is a British philosopher. His areas of interest are the history of philosophy, philosophical logic, and the theory of identity.He was educated at Balliol College, Oxford...

, and David Wiggins

David Wiggins

David Wiggins is a British moral philosopher, metaphysician, and philosophical logician working especially on identity and issues in meta-ethics. His 2006 book, Ethics. Twelve Lectures on the Philosophy of Morality defends a position he calls "moral objectivism".- Life :Wiggins read philosophy...

. 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."