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.
Motivation for PRO There are several independent pieces of linguistic theory which motivate the existence of PRO. The following four are reviewed here:
the extended projection principle the theta criterion binding theory nominal agreement
Extended projection principle 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 grammaticality 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 [TP to 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 [TP to sleep] co-refers with Bill.
Since the argument that controls PRO in (1a) is the subject, this is called subject control, and PRO is co-indexed with its antecedent John, As shown in (2a), it is also possible to have object control, where the argument that controls PRO is the object of the main clause, and PRO is co-indexed with its antecedent Bill. In the context of the EPP, the existence of subject and object control follows naturally from the fact that the null pronominal subject PRO can be co-indexed with different DP arguments. While (1a) and (2a) show the surface sentences, (1b) and (2b) show the more abstract structure where PRO serves as the subject of the non-finite clauses, thereby satisfying the EPP-feature of T (realized by infinitival 'to'). The following tree diagrams of examples (1) and (2) show how PRO occupies the subject position of non-finite clauses.
Theta criterion Every verb has theta roles and under the theta criterion every theta role must be present in the structure of the sentence; this means that theta roles must be associated with a syntactic position even when there is no overt argument. Therefore, in the absence of an overt subject, the null category PRO helps to satisfy the theta criterion. For example:
In example (3), the verb examine is associated with the following lexical entry:
examine: V <DPagent DPtheme>
Accordingly, the verb examine must have a DP (determiner phrase) as an agent and a DP as a theme. However, in (3a), since no overt DP functions as the agent of examine, this should be a violation of the theta criterion. However, the presence of the null PRO subject, as shown (3b), satisfies the theta criterion by having PRO as the DPagent in the sentence and the patient as the DPtheme. The tree diagram (3) represents how PRO satisfies the theta criterion of examine by being the DPagent in the non-finite clause.
Binding theory
The claim that non-finite clauses have a phonologically null PRO subject is in part motivated by binding theory — in particular, the idea that an anaphor requires a local antecedent to be present. Reflexive pronouns such as myself and oneself require a local antecedent. As shown in (4), PRO can function as an antecedent for reflexives: in (4a) PRO is the antecedent for the reflexive pronoun 'myself', and in (4b) PRO functions as the antecedent for the impersonal reflexive oneself. If the null subject PRO were not present in examples like (4a) and (4b), then non-finite clauses would contain anaphors that lacked a local antecedent, and incorrectly predicting that such sentences to be ungrammatical. The grammaticality of such sentences confirms that the reflexives have an antecedent, which by hypothesis is PRO.
Note, however, that PRO itself has no local antecedent in these examples: PRO can share reference with an external referent as in (4a), or have an arbitrary reading as in (4b).
Nominal agreement Evidence that non-finite clauses have a phonologically null PRO subject comes from the fact that predicate nominals must agree with the subject of a copular clause. This is illustrated in (5) and (6). Example (5) shows that the number of the predicate nominal must agree with that of the overt subject: in (5a) the singular subject (their son) requires a singular nominal predicate (millionaire); in (5b), the plural subject (his sons) requires a plural nominal predicate (millionaires). The examples in (6) show that the same contrast holds of PRO subjects: if PRO is controlled by a singular antecedent, in (6a) the subject of want, then the predicate nominal must be singular; if PRO is controlled by a plural antecedent, as in (6b), then the predicate nominal must be plural.
The following tree diagrams show how PRO, as the subject of the copular clause, enters into agreement with the nominal predicate introduced by the copular verb become. The application of agreement is automatically explained if PRO is co-indexed with the subject of the main clause, with the predicate nominal simply agreeing with the number features of the argument that controls PRO, just as it would be if an overt subject had been introduced.
Theoretical status of PRO There are two main approaches to PRO:
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