PRO (linguistics)

In generative linguistics, PRO (called "big PRO", distinct from pro, "small pro" or "little pro") is a pronominal determiner phrase (DP) without phonological content. As such, it is part of the set of empty categories. The null pronoun PRO is postulated in the subject position of non-finite clauses. One property of PRO is that, when it occurs in a non-finite complement clause, it can be bound by the main clause subject ("subject control") or the main clause object ("object control"). The presence of PRO in non-finite clauses lacking overt subjects allows a principled solution for problems relating to Binding Theory.

Within Government and Binding theory, the existence and distribution of PRO followed from the PRO Theorem, which states that PRO may not be governed. More recent analyses have abandoned the PRO Theorem. Instead, PRO is taken to be in complementary distribution with overt subjects because it is the only item that is able to carry null case which is checked for by non-finite Tense Markers (T), for example the English to in control infinitives.

There are several independent pieces of linguistic theory which motivate the existence of PRO. The following four are reviewed here:

The Extended Projection Principle (EPP) requires that all clauses have a subject. A consequence of the EPP is that clauses that lack an overt subject must necessarily have an "invisible" or "covert" subject; with non-finite clauses this covert subject is PRO.

Motivation for a PRO subject comes from the grammatical of sentences such as (1) and (2), where the subject of the infinitival to-clause, though not overtly expressed, is understood to be controlled by an argument of the main clause. In (1a), the subject of control is understood to be the same person that issued the promise, namely John. This is annotated in (1b) by co-indexing John with PRO, which indicates that the PRO subject of [_TPto control the situation] co-refers with John. In (2a), the subject of sleep is understood to be the same person that was convinced, namely Bill. This is annotated in(2b) by co-indexing Bill with PRO, which indicates that the PRO subject of [_TPto sleep] co-refers with Bill.

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