The Extended Projection Principle (EPP) is a linguistic hypothesis about the obligatoriness of subjects. It was proposed by Noam Chomsky as an addendum to the Projection principle (which it has outlived significantly). The basic idea of the EPP is that clauses must contain an NP in the subject position (i.e. in the specifier of TP or IP, or in the specifier of VP in languages in which subjects don't raise to TP/IP such as Welsh).
Most verbs require meaningful subjects—for example, "kick" in "Tom kicked the ball" takes the subject "Tom". However, other verbs do not require (and in fact, do not permit) meaningful subjects—for example, one can say "it rains" but not "the sky rains". The EPP states that regardless of whether the main predicate assigns a meaningful theta role to a subject, a subject must be present syntactically. As a result, verbs which do not assign external theta roles will appear with subjects that are either dummy pronouns (e.g. expletive "it," "there"), or ones which have been moved into subject position from a lower position (e.g. subject of an embedded clause).
Examples which have been proposed to be the result of expletive subject insertion in accordance with the EPP:
- It seemed that John would never calm down.
- It ( rains | snows | hails | etc. ) frequently in Quebec.
- There seems to be a problem with the radiator.
Notice that in all of these the overt subject has no referential reading.
In languages that allow pro-drop (such as Spanish or Italian), the empty category pro (not to be confused with PRO) can fulfill the requirement of the EPP.
McCloskey (1996) proposed that there is one group of languages that lacks the EPP: the VSO languages (like Irish), which appear not only to lack expletives, but also to lack movement operations triggered by the EPP.
In his dialogue De ente, Plato states that there cannot be a sentence without a noun and a verb. Aristotle teaches the same in Peri hermeneias 2: 'Thus, a verb without a subject will mean nothing'. -El Brocense, 1587. This is an early formulation of Chomsky's Extended Projection Principle.
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