PRO (linguistics)

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PRO (pronounced big pro, to distinguish it from pro, pronounced little/small pro) is an empty category whose existence is first postulated in classical Government and Binding Theory. There are two independent pieces of evidence for its existence: the Extended Projection Principle (which states that all clauses must have a subject) and theta criterion (which states that every argument a verb can assign must be realized):

  • I persuaded John [PRO to read Al Gore's latest book]

In the example above, PRO serves as the subject of the non-finite clause [to read Al Gore's latest book], thereby satisfying the EPP-feature of T (occupied by the infinitival 'to' in the example). Since it is an object control sentence, 'John' is the antecedent of PRO, they are co-indexed.

The interpretation of PRO may be either dependent on another noun phrase (like anaphors) or arbitrary (like pronominals). That is why in terms of features PRO may be described as [+anaphor, +pronominal]. However, this set of features would pose a problem for Binding Theory: an anaphor must be bound in its governing category, whereas a pronominal must be free (in its governing category). The solution that Chomsky proposed was the so-called PRO Theorem: PRO must be ungoverned. In other words, PRO cannot be governed and that is why it cannot have a governing category.

In recent literature (Chomsky 1999, Radford 2004) the existence of PRO has been argued for without the help of the PRO Theorem and government theory. It is posited that PRO is in complementary distribution with overt subjects because it is the only item that is able to check "null case" from a nonfinite T, namely the 'to' in control infinitives. By positing this, the problems arising from the PRO Theorem are avoided. The problem was that PRO serves as the subject of the control clause and must, as such, be case-marked. However, following the traditional G&B-framework, case-marking only applies under government. Thus, PRO could not be case-marked since it must be ungoverned. If PRO receives null-case, it is licensed to appear in subject position.

On the other hand, Norbert Hornstein (1999) has proposed that control verbs can be explained without resort to PRO, and that PRO can be done away with entirely.


  • Martin, Roger. 2001. Null Case and the Distribution of PRO. Linguistic Inquiry 32(1), 141–166.
  • Hornstein, Norbert. 1999. Movement and Control. Linguistic Inquiry 30(1), 69–96.