SYNTAX: pronoun without phonetic properties which is the subject of (e.g.) infinitivals. EXAMPLE: in (i) PRO is the subject of to win.
(i) John tried [ PRO to win]The fact that John is understood to be the one that should be winning is captured (in Control Theory) by assuming that PRO must be coreferential with John. The distribution of PRO was assumed in Chomsky (1981) to follow from its Binding Theoretic properties. Since PRO is lexically specified as being both +anaphoric and +pronominal, it must obey both condition A and condition B of the Binding Theory. The resultant contradictory requirements (PRO must be both bound and free within its binding domain) can be met only if PRO has no binding domain by virtue of not having a governor (the "PRO theorem"). This explains the illformedness of (ii) and (iii), where PRO is governed (by kissed and tensed INFL respectively), as opposed to (i), under the assumption that whether or not the subject position in a clause is governed depends on whether the sentence is tensed.
(ii) * John kissed PRO (iii) * John promised [ that PRO would win ]
|LIT.||Chomsky, N. (1986a)|
Chomsky, N. (1981)