COGNITIVE SCIENCE 2, 391-396 (1978)

A Conceptual Analysis of the Verbs Need and Want*

ROEERT WILENSKY University of California at Berkeley

1. THE PROBLEM

In spite of efforts to represent and use intentional knowledge (see Schank & Abelson, 1977, and Schank & Wilensky, 1977), some common English words involving intentions have remained resistant to conceptual analysis. Consider the problem of finding definitions of the verbs needand want that can be used for natural language understanding and generation. The following examples indicate some of the features that such a representation must have:

(1) John wanted some money. (2) John needed some money.

Most readers of (1) interpret it to mean that John had the goal of possessing some money. While this is also plausible in (2), in that sentence we assume that having money is not a goal in itself, but that it is a precondition for some other, unstated goal. However, the fact that something fulfills a precondition is not a sufficient definition for need. For example, suppose we had represented in our memory a conceptualization denoting that John was hungry, and that he was trying to get a hamburger in order to eat it. While we might want to generate

(3) John wanted a hamburger.

from this structure, it would be odd to generate

(4) John needed a hamburger.

even though eating or possessing a hamburger would fulfill John's goal or satisfy a precondition for it, respectively.

If, however, we had previously learned that someone promised John a thousand dollars if he brought him a hamburger within three minutes, it

*The research described here was done at the Yale Artificial Intelligence Project and is funded in part by the Advance Research Projects Agency of the Department of Defense and monitored under the Office of Naval Research under contract N00014-75-C-1111. Requests for reprints should be sent to Robert Wilensky, Department of Electrical Engineering and Computer Science, Computer Science Division, University of California, Berkeley, Berkeley, California 94720.

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would then be reasonable to generate sentence (4). One apparent difference between this situation" and the previous case is that here "possessing a hamburger" does not just meet a precondition for a plan; it is that precondition. That is, it is somehow the "essentially unique" way of fulfilling the precondition. If we could formalize this notion, it would also explain why understanding (2) is not troublesome--it is easy to envision goals for which money is an unsubstitutable prerequisite.

However, first consider the story.

(5) John had been wandering in the desert for a long time. (6) John needed a drink of water.

Most readers interpret (6) to mean that John will become ill if he does not get some water soon. However, this does not appear to be a particularly unique prerequisite for preventing dehydration; a glass of orange juice would probably do as well. Likewise,

(7) John needed a shot of heroin.

is a reasonable sentence to generate from a representation describing that John was a junkie, and had the goal of preventing withdrawal. We would not want to generate (7) if John merely wanted to get high, because it is possible to get high in other ways. But as in the previous example, shooting heroin is really not a unique way to stave off addiction, since morphine or methadone would do as well. Why, then, can we generate need for the goal "prevent withdrawal," but not for the goal "get high"?.

Furthermore, while it is often appropriate to generate want instead of need in the above examples, this is not always the case. For example, suppose we had in memory a representation denoting that John had the goal of paying a parking ticket and attached to that goal the sequence of plans John went through to get the ticket paid. In generating English for this story, we might begin with

(8) John needed to pay a parking ticket.

but it would be inappropriate, or at least unusual, to generate

(9) John wanted to pay a parking ticket.

Similarly, if a person armed with a knife attacked John, and John subsequently had the goal of preserving his life, we might generate the sentence

(I0) John needed to defend himself.

but it would be inappropriate to use

(11) John wanted to defend himself.

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2. A SOLUTION

We would like to propose a definition for need and want that accounts for the various uses of each word outlined above, and which reflects the differences between the two words. This definition presupposes the following characterizations of intentional knowledge. First, we need a unit of intentionality called a goal that is simply a mental state that leads to actions mediated by thoughts. Second, each goal has asource or reason that that goal came into being. There are two kinds of sources:

I. Plan Source. The goal has come into existence because it is an instrumental goal, i.e., a goal aimed at fulfilling a precondition for a plan for another goal. For example, having some food may be instrumental to the plan to satisfy hunger by eating.

2. Thematic Source. A goal arises in the first place because of certain basic tendencies that we call themes. For example, people have drives like hunger that periodically give to goals. In addition, a PRESERVATION theme may give rise to the goal of saving one's life if an event occurred which threatened it; an ENJOYMENT theme can give rise to a goal of doing something from which one gets pleasure; an OBLIGATION theme can give rise to a goal to repay a social debt.

Themes are generally useful in natural language understanding because they can predict and explain the goals that characters in a text are likely to have.

It is now possible to make the following definition of want and need."

NEED: "A needs X" is used when a. X is an essentially unique precondition for a plan for a goal of A's, or b. X is an essentially unique action that would result in a goal of A's.

WANT." "A wants X" is used when A has X as a goal, and the source of X is other

than a PRESERVATION or OBLIGATION theme.

We must now provide an adequate definition of"essential uniqueness." For this, we propose the following: State S is essentially unique to precondition P if

I. S is identical to P. 2. S is the only state that meets the requirements of P that is "feasible"

under the circumstances. 3. S mentions an object that is the "prototypical object" meeting the

requirements of P.

The concept of uniqueness for plans is entirely analogous.

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By "feasibility" we mean that, in the judgment of the natural language processor, the assessed cost is not much greater than the assessed value. Thus, if a person dying of thirst in the desert came across someone drinking a bottle of Coke, we would permit him to say "Please, I need a drink of Coke," because under the circumstances, drinking Coke appears to be the only feasible action. An intuitive system for computing the costs and values of goals and actions is discussed in Schank and Abelson (1977).

The notion of "prototypical object" is essentially the notion of a default object for a particular purpose. For example, attached to goal "prevent withdrawal from heroin" in our memory is the plan "shoot narcotic," and attached to this plan, that the prototypical narcotic is heroin. Similarly, associated with the plan of hammering something is that the prototypical hammering object is a hammer. The claim here is that even though it is possible to use one's shoe as a hammer, "shoe" would not be listed along with hammer as the objects to select from when performing the hammering action.

Let us apply this definition to some of the previous cases. Suppose we had a structure in memory that represented John having the goal "possess heroin," the source of which was the plan "ingest narcotic" for the goal "prevent withdrawal." Since heroin is listed in memory as the prototypical object for this function, we can generate "John needed some heroin." However, if instead the "possess heroin" goal stemmed from a goal to "get high," then we would not generate need because having heroin is not an essentially unique precondition to "get high," for most natural language processors. However, since the goal of "getting high" is neither obligational nor preservational, by our definitions above we could generate "John wanted some heroin." Furthermore, if we had the same goal structure, but we knew that the only drug available was heroin, then "John needed some heroin" is allowable by the feasibility condition of essential uniqueness. And in any case, we could generate "John needed some drugs" since this meets the identity condition.

Suppose now we are trying to understand the sentence "John needed some heroin." In accordance to the definition above, we should ask if "possess heroin" is the unique precondition for a plan for some goal. Once again, since heroin is the prototypical object in the plan for the goal "prevent withdrawal," we infer that this indeed is John's goal and plan. However, if we heard this sentence after the sentence "John was a drug pusher," we could infer that John had the goal of possessing heroin for the purposes of selling it. In this context, "possessing heroin" is an essential precondition of the plan for making money by selling heroin. Thus, we would conclude that John was out of heroin and needed some more in order to sell it to his customers. Thus, our definition helps us in understanding by providing a representation from which we can make salient inferences.

Note that our model also predicts that the understander would have trouble with a sentence like "John needed a hamburger." The functional uses

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associated with hamburger would presumably be "cause enjoyment via eating" and "satisfy hunger via eating." Since "possess hamburger" is not an essentially unique precondition for either of these, both goals are rejected, and the sentence appears confusing. However, if the sentence were " John needed some food," then since "possess food" is an essentially unique precondition of these goals, as well as the goal "prevent s tarvation," the sentence is understandable.

As for want, suppose we were trying to understand the sentence " John wanted some food." Because want precludes a goal source that is a preservation theme, we can rule out the possibility that John was starving, and infer that he had this goal because he was either hungry or wished to enjoy eating. Also, in the situations described above, for which it was reasonable to generate "John needed to defend himself" and "John needed to pay a parking ticket," both contained goals whose source was the preservation theme, and according to the rule above, we can rule out generating want in both these cases.

According to these rules, there are intentional situations in which neither want nor need are applicable. For example, if John had promised Mary he would repay a previous favor to her by helping her move, then it would not be appropriate to generate

(12) John needed to help Mary move.

since the goal of helping Mary is not instrumental to anything, nor should we generate.

(13) John wanted to help Mary move.

since the goal has an O B L I G A T I O N theme. We can, however, generate the form have to verb, as in

(14) John had to help Mary move.

(I am indebted to Eugene Charniak for this example). We can define this usage of have as follows: have to verb can be used in all the situations in whic_h need can be used, as well as in the case of a goal whose theme is obligational.

One further difference between want and need should now be made explicit. Consider

(15) John needed a new umbrella, but he didn't realize it until he got drenched.

(16) John wanted a new umbrella, but he didn't realize it until he got drenched.

(16) is clearly an absurd sentence, while (15) is readily understandable. The reason for this is that need specifies that a state is a prerequisite for a goal,

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without specifying that it is actually an instrumental goal of the planner. That is, in the case of need that the thing needed is a goal is an inference, whereas for want, it is part of the meaning of the word.

3. CONCLUSION

Our goal was to formulate definitions of the words want and need that could be used by a natural language processor. In doing so, we found it necessary to postulate the existence of memory structures and processes that can be used to retrieve and identify prototypical objects for a plan, calculate the cost and value of achieving a goal, and determine why a goal has come into existence. Thus, the problem of representing the meanings of natural language utterances is intimately related to the problem of the structure of knowledge and its organization in memory; as our understanding of the nature of a particular kind of knowledge increases, our ability to represent the meaning of natural language utterances follows in its wake.

REFERENCES

Schank, R. C., & Abelson, R. P. Scripts, plans, goals, and understanding. Hillsdale, N.J.: Lawrence Erlbaum Press, 1977.

Schank, R. C., & Wilensky, R. A Goal Directed Production System for Story Understanding. In the Proceeding of the Workshop on Pattern Directed Inference Systems. Honolulu, Hawaii, 1977.