Editing X-bar theory (section)

== Background ==
The X-bar theory was developed to resolve the issues that [[phrase structure rules]] (PSR) under the [[Standard Theory]]<ref name="C65">{{Cite book |last=Chomsky|first=Noam|authorlink= |title=Aspects of the Theory of Syntax|publisher=MIT Press|location=Cambridge, MA|language= |year=1965|page= |id= |isbn= |quote= }}.</ref> had.<ref name="chom-dict-xbar">{{Cite book|author=Haraguchi|display-authors=et al.|authorlink= |translator= |title=Zouhoban Chomsky Riron Jiten|publisher=Kenkyusha|location=Tokyo|year=2016|pages=521–523|id= |isbn= |quote= |edition= Kenkyusha's Dictionary of Theoretical Linguistics Enlarged}}</ref>

The PSR approach has the following four main issues.
# It assumes [[exocentric structure]]s<ref group="FN">Phrasal structures that lack a [[head (linguistics)|head]].</ref> such as "S → NP Aux VP". This is contrary to the fact that [[phrases]] have [[head (linguistics)|heads]] in all circumstances.<ref name="chom-dict-xbar" />
# While the sentence ''John talked to the man'', for example, involves the PSR of a [[verb phrase]] "VP → V (PP)",<ref group="FN">The parentheses indicate that the [[phrasal category]] in them is optional.</ref> ''John talked to the man in person'' involves the PSR of "VP → V (PP) (PP)". This indicates that it is necessary to posit new PSRs every time when an undefined structure is observed in [[E-language]], which amounts to adding an indiscriminate number of grammatical rules to [[Universal Grammar]]. This poses serious issues from the perspectives of the [[Plato's problem]] and the [[poverty of the stimulus]].<ref name="C65" />
# It wrongly rules in structures that are impossible in [[natural language]] such as "VP → NP A PP", because as in 1 and 2, the PSR countenances phrases that do not have [[endocentric structure]]s.<ref group="FN">Phrasal structures that include a [[head (linguistics)|head]].</ref><ref name="chom-dict-xbar" />
# It fails to capture sentence ambiguities because it assumes flat, nonhierarchical structures.<ref name="basic-GG">{{Cite book|last=Kishimoto|first=Hideki|authorlink= |title=Basic Seisei Bunpo (The Basics of Generative Grammar)|publisher=Hitsuji Shobo|location=Tokyo|language= |year=2009|page=|id= |isbn= |quote= }}</ref>
The X-bar theory is a theory that attempts to resolve these issues by assuming the mold or template phrasal structure of "XP".