• They consist of various relations and variables/slots, and values for these variables.

• The relations can take a variety of forms; they can be simple relations (e.g. is-a, hit, kick) or they can be more complex, "causal" relations (e.g. enable, cause, prevent, desire).

• Variables/slots contain concepts or other sub-schemata; any concept that fills a slot usually has to satisfy some test (e.g. the argument-slot "Agent" in the relation HIT [Agent, Object, Instrument] requires that the concept that fills it is an animate object).

• Values refers to the various specific concepts that fill or instantiate slots.

• Schemata, thus, encode general or generic knowledge that can be applied to many specific situations, if those situations are instances of the schema; for example, the HIT relation could characterise a domestic dispute (e.g. Harry hit the child) or a car crash (e.g. the van hit the lorry).

• Schemata can often leave slots "open" or have associated with them default concepts that are assumed if a slot is unfilled; for Instance, we are not told what instrument Harry used (in "Harry hit the child"), but we tend to assume a default value (like a stick or a hand).

to understand commonplace events like going to a restaurant.

Schank and Abelson were interested in capturing the knowledge people use to comprehend extended texts, like the following one:

Ruth and Mark had lunch at a restaurant today. They really enjoyed the meal but were worried about its cost. However, when the bill arrived after the ice cream, they were pleasantly surprised to find that it was very reasonable.

In reading this passage, we use our knowledge to infer that the meal (mentioned in the second sentence) was at the restaurant where they had lunch (mentioned in the first sentence), that the meal involved ice-cream and that the bill did not walk up to them but was probably brought by a waiter. Schank and Abelson argued that we must have predictive schemata to make these inferences and to fill in aspects of the event that are left implicit. The specific schemata they proposed were called scripts. Scripts are knowledge structures that encode the stereotypical sequence of actions in everyday happenings. For example, if you often eat in restaurants then you would have a script for "eating in restaurants". This "restaurant script" would encode the typical actions that occur in this scenario along with the sorts of objects and actors you would encounter in this context. The restaurant script proposed by Schank and Abelson had four main divisions: entering, ordering, eating, and leaving. Each of these general parts had sub-actions for what to do: for instance, entering breaks down into walking into the restaurant, looking for a table, deciding where to sit, going to a table, and sitting down (see Table 9.4).

Within this schema the relations are the various actions, like walking or sitting. The slots in the script are either roles (e.g., waiter) or headings for other sub-schemata (e.g., entering). Role slots capture the various

"parts" in the script like the waiter, the customers and the cook, and are filled by the specific people in the situation (e.g., the tall waiter with the receding hairline). Ordinarily, these roles can only be filled by an object that satisfies the test of being human (e.g., a waiter who is a dog is unexpected and extraordinary). The general components of the script (e.g., entering, ordering) are different types of slots that contain subschemata (concerning the various detailed actions of walking, sitting and so on). In this way, it is possible to create structures that characterise people's knowledge of many commonplace situations.

Evidence for script theory

Several studies have investigated the psychological plausibility of scriptal notions (see Abelson, 1981; Bower, Black, & Turner, 1979; Galambos, Abelson, & Black, 1986; Graesser, Gordon, & Sawyer, 1979; Sanford & Garrod, 1981; Walker & Yekovich, 1984). Bower et al. (1979) asked people to list about 20 actions or events that usually occurred when eating at a restaurant. In spite of the varied restaurant-experiences of their subjects,

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