Notes on computational linguistics.pdf - UCLA Department of ...

Stabler - Lx 185/209 2003
It is important to reflect on what this view must amount to. A competent language user must not only
be able to perform the syntactic analysis, but also must have the inference rules that are defined over these
analyses. This is an additional and significant requirement on the adequacy of our theory, one that is only
sometimes made explicit:
For example, trivially we judge pretheoretically that 2b below is true whenever 2a is.
2a. John is a linguist and Mary is a biologist.
b. John is a linguist.
Thus, given that 2a,b lie in the fragment of English we intend to represent, it follows that our system
would be descriptively inadequate if we could not show that our representation for 2a formally
entailed our representation of 2b. (Keenan and Faltz, 1985, p2)
Spelling out the idea that syntax defines the structures to which inference applies, we see that syntax is much
more than just a theory of word order. It is, in effect, a theory about how word order can be a reflection of
the semantic structures that we reason with. When you learn a word, you form a hypothesis not only about
its positions in word strings, but also about its role in inference. This perhaps surprising hypothesis will be
adopted here, and some evidence for it will be presented.
So we have the following views so far:
• semantic values and entailment relations are defined over syntactic derivations
• linguistic theory should explain the recognition of entailment relations that hold in virtue of meaning
Now, especially if the computational model needs the inference relations (corresponding to entailment) but
does not really need the semantic valuations, as noted at the beginning of this section, this project may sound
easy. We have the syntactic derivations, so all we need to do is to specify the inference relation, and consider
how it is computed. Unfortunately, things get complicated in some surprising ways when we set out to do this.
Three problems come up right away:
First: Certain collections of expressions have similar inferential roles, but this classification of elements according
to semantic type does not correspond to our classification of syntactic types.
Second: Semantic values are fixed in part by context.
Third: Since the syntax is now doing more than defining word order, we may want to modify and extend it for
purely semantic reasons.
We will develop some simple ideas first, and then return to discuss these harder problems in §16. We will
encounter these points as we develop our perspective.
So to begin with the simplest ideas, we will postpone these important complications: we will ignore pronouns
and contextual sensitivity generally; we will ignore tense, empty categories and movement. Even with
these simplifications, we can hope to achieve a perspective on inference which typically concealed by approaches
that translate syntactic structures into some kind of standard first (or second) order logic. In particular:
• While the semantics for first order languages obscures the Fregean idea that quantifiers are properties
of properties (or relations among properties, the approach here is firmly based on this insight.
• Unlike the unary quantifiers of first order languages – e.g. (∀X)φ – the quantifiers of natural languages
are predominantly binary or “sortal” – e.g.
quantifiers.
every(φ, ψ). The approach adopted here allows binary
• While standard logic allows coordination of truth-value-denoting expressions, to treat human language
we want to be able to handle coordinations of almost every category. That is not just
human(Socrates) ∧ human(Plato)
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