## Introduction to Artificial Intelligence: Problem Set 4

Assigned: Feb. 12
Due: Feb 21 (Feb. 19 is a holiday)

## Problem 1:

For each of the ambiguities listed below, explain how it can be disambiguated using either selectional restrictions, frequency in context (for lexical ambiguity), or world knowledge. If disambiguation is possible using selectional restrictions, you should prefer this to the other two, as selectional restrictions are usually both stronger and easier to implement. If you use selectional restriction, explain what are the features, the relation, and the constraint involved. If you use frequency in context, explain how the context is established by other parts of the sentence. If you use world knowledge, explain the rules being used. For example:

• "The bat ate its dinner" [= flying mammal rather than wooden club]. Answer: Selectional restrictions. The subject of ``ate'' must be ANIMATE: ``wooden club'' is not ANIMATE.
• ``I went up to the door of the house with the red shutters'' [``with the red shutters'' is attached to ``house'' not ``door'' or ``went''.] Answer: World knowledge. Shutters are part of a house, not part of a door. It is unusual to go places carrying shutters.
• ``In the seventh inning, the pitcher was replaced''. [``pitcher'' = baseball player rather than container.] Answer: Frequency in context. The word ``inning'' establishes the context as baseball.

Problems:

• A. President Bush proposes to cut taxes on inheritances because they are too high. ("proposes" = "formally suggests" rather than "offers marriage"; "cut" = reduce rather than literally slice; "they" = taxes rather than inheritances; "high" = a large amount of money rather than literally elevated.)

Answer: "proposes" can be disambiguated using frequency in context. In a political context (established by "President Bush" and "taxes"), the meaning "proposes marriage" is rare. "cut" can be disambiguated using selectional restrictions. The meaning "literally slice" takes a physical object as its object, not an abstract objects such as "taxes". The word "high" can be disambiguated by selectional restrictions. Only entitities with a physical location can be elevated, whereas neither "taxes" or "inheritances" characteristically have a physical location. Disambiguating "they" requires reasoning that a reason for reducing X is that X is too large. Therefore, the thing that is specified as the object of "reduce" (taxes) is the same thing that is too high.

• B. Emily cuts Anne whenever they meet, because she was once noisily drunk at Emily's dinner party. ("cuts" = deliberately ignores. "she" = Anne. "was ... drunk" = "was intoxicated" rather than "was imbibed.")

Answer: The word "cuts" can be disambiguated using world knowledge; it is possible for one person to physically cut another once or twice, but it would be peculiar for this to happen habitually. The word "was drunk" can be disambiguated using selectional restrictions; only a liquid can be imbibed. The disambiguation of "she" requires the world knowledge that X being noisily drunk at Y's party is a social offense that may result in Y snubbing X.

• C. Sam hurt Joe with a baseball bat. ("hurt" = "caused pain" or "injured")

Answer: Selectional restictions on the instrumentality of a physical object "baseball" suggest the meaning "caused pain". Another selectional restriction states that the actor of "caused pain" must be animate.

• D. The baseball bat hurt Joe. (Why would it important in semantic processing to distinguish this from the meaning in sentence (C)? What disambiguation strategy can be used?)

Answer: It is important to recognize that here "baseball bat", the grammatical subject of "hurt" is the instrument of the action "cause pain", whereas in sentence (C) "Sam", the grammatical subject, is the actor. This can be derived by actor. This can be derived by selectional restrictions; the actor must be animate.

• E. Joe's head hurts him. ("hurts" = "is in pain")

Answer: World knowledge: It is less likely that a person's own body part is causing them pain than that it is in pain. There are exceptions e.g. in "Joe's ingrown toenail hurts him" the toenail is not in pain, it causes pain to the toe.

• F. Sam hurt Joe with his rude remarks about his clothes. ("hurt" = "upset the feelings of"; first "his" = Sam. second "his" = Joe.)

Answer: The word "hurt" can be disambiguated using selectional restrictions on the instrument "with ... remarks"; the instrument of "cause physical pain" must be a physical object, not remarks. The disambiguation of the two "his"'s requires world knowledge. The first "his" can be disambiguated using the rule that, if X effects a result using speech, it is almost always using X's own speech, rather than someone else's speech. The disambiguation of the second "his" is really quite difficult; one needs some substantial body of knowledge about typical insults to realize that it is more likely for X to insult Y by talking about Y's clothes than by talking about X's own clothes.

One disambiguation here that we assumed, but did not justify is the synactic disambiguation of "with ... remarks" as giving an instrumentality for "hurt" rather than attaching to Joe. That can perhaps be justified purely on grounds of grammatical frequency; attaching prepositional phrases to a proper noun is comparatively rare, though certainly possible.

• G. Joe hurt his chances of getting the job by showing up an hour late to the interview. ("hurt" = diminished.)

Answer: Selectional restrictions: Neither "caused pain" nor "insulted" can take an abstraction like "chances" as object.