Polarity item

In linguistics a polarity item is an expression which is sensitive to the presence, in the same sentence, of certain other expressions, known as "licensing" (or "anti-licensing") expressions.

The most well-known polarity items are those that are sensitive to negation and related expressions. These polarity items divide into those that must co-occur with a "somehow negative" expression ("negative polarity item", NPI) and those that cannot ("positive polarity item", PPI). An example of an NPI is the English word any. It is ungrammatical if it occurs in a sentence without a "negative" expression (A star "*" in front of a sentence means the sentence is believed to be ill-formed):
 * John doesn't have any potatoes.
 * *John has any potatoes.

One says of an NPI like any that it is licensed by a negative expression. NPIs are also usually licensed by questions, as in:
 * Does John have any potatoes?

An example of a PPI is the English word somewhat. If it occurs in a sentence with another negative expression, the sentence is ill-formed:
 * John liked it somewhat.
 * *John didn't like it somewhat.

One says of a PPI like somewhat that it is anti-licensed by a negative expression. PPIs are also not generally permitted in questions:
 * *Did John like it somewhat?

Early discussion of polarity items can be found in the work of Otto Jespersen and Edward Klima.

Much of the research on polarity items has centered around the question of what it takes for an expression to be "somehow negative". In the late seventies, William Ladusaw (building on work by Gilles Fauconnier) discovered that most NPIs are licensed in downward entailing environments. This is known as the Fauconnier-Ladusaw Hypothesis. A downward entailing environment, however, is not a necessary condition for an NPI to be licensed - they may be licensed by some non-monotone (and thus not downward entailing) contexts, like "exactly N", as well.
 * *Some people have ever been on the moon.
 * Exactly three people have ever been on the moon.

Licensing contexts may be n-words (negative particles, negative quantifiers), antecendent of conditionals, questions, restrictor of universal quantifiers and superlatives, non-affirmative verbs (doubt), adversative predicates (be surprised), neg raising verbs (believe), negative conjunctions (without), comparative than-sentences, too-comparatives, negative predicates (unlikely) and others (finally, only).

Different NPIs may be licensed by different expressions. Thus, while the NPI anything is licensed by the downward entailing expression at most two visitors, the idiomatic NPI lift a finger is not licensed by the same expression.
 * At most two of the visitors had seen anything.
 * *At most two of the visitors lifted a finger to help.