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<Paper uid="P90-1025">
  <Title>THE LIMITS OF UNIFICATION</Title>
  <Section position="3" start_page="0" end_page="0" type="intro">
    <SectionTitle>
1 Introduction
</SectionTitle>
    <Paragraph position="0"> In recent years, a great deal of attention has been devoted to complex-feature based grammar formalisms-i.e. grammar formalisms in which syntactic elements are not atomic symbols, but rather complex elements, such as value-attribute or term structu~s; see Shieber (1986) for an overview. Typically such formalisms use a single mechanism--variable substitution--for all purposes, and the most widely used variable substitution mechanism is unification) Such complex-feature based grammars, then, axe viewed as systems in which partial feature structures are built up, by the process of unification, into successively more specified structures. While it is formally elegant to use a single mechanism for a number of purposes, this theoretical elegance is realized in practice only if the mechanism does not require the other modules of the system to be complicated to achieve this &amp;quot;elegance&amp;quot;. Currently, unification is used for at least four puq3oses: 1 In the rest of this paper, for convenience I will use the term &amp;quot;unification&amp;quot; instead of &amp;quot;variable substitution&amp;quot;, since it is the most commonly used type of variable substitution, but it should be borne in mind that the point being made here holds for variable substitution, in general.</Paragraph>
    <Paragraph position="1"> * to enforce formal agreement between purely syntactic features * to &amp;quot;percolate&amp;quot; features between a pre-terminal category and the phrase which it heads * to pass features between a dislocated element-such as a WH-phrase--and its trace * to build up semantic representations This paper will focus on the use of unification to enforce agreement and will present evidence from several natural languages which argues against its use in the case of purely formal syntactic features: when such features are lexically or morphologically underspecified, they remain so, even under agreement, contrary to the predictions of a system using unification for agreement.</Paragraph>
    <Paragraph position="2"> Moreover, it is worthwhile stressing at the outset that the main argument of this paper is not that there are certain constructions that present a problem for unificalion, and, hence, require some technical solution. The point is much stronger:, even if some elaborate analysis can be devised that allows unification to be used to effect agreement, this would be the wrong tack to take.</Paragraph>
    <Paragraph position="3"> Rather, the argument will go, using unification to effect agreement is incorrect both for theoretical reasons--it presents a view of language which is contradicted by the facts--and for practical reasons---using unification to effect agreement can impede a system's robustness and transportability.</Paragraph>
  </Section>
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