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In linguistics, a theta role or θ-role is the semantic role a noun phrase plays in a sentence. As such it is a semantic rather than a syntactic feature, in contrast to such notions as the subject of a sentence or a prepositional object.
For instance, in the sentence Billy ate the cake, "Billy" is both the subject of the sentence and the agent and "the cake" is the object of the verb and the patient. But in The cake was eaten by Billy, "Billy" is still the agent, even though "the cake" is now the subject of the sentence.
Major theta roles
Here is a list of the major theta roles.
- The agent is whoever is intentionally carrying out some action. In Joe hits the ball, Joe is the agent.
- The experiencer is someone who experiences some state. Thus, in Jack fell asleep, Jack is the experiencer. This is because Jack is not an agent, in that he did not "fall himself asleep". One semantic test used to distinguish the two roles is to ask if "I promise to..." makes sense. E.g., "I promise to hit the ball" versus ?"I promise to fall asleep".
- The patient is whatever is acted on. Thus in Debra broke the window, the window is the patient.
- The instrument is whatever is being used to perform the action. In I hit the ball with a bat, the bat is the instrument.
Other theta roles exist in the literature, but many, such as theme, tend to be more controversial.
Relationship of syntax to theta roles
In languages such as English which rely heavily on word order and use frequent passivization, identification of theta roles from merely syntactic cues is often impossible. In more heavily case-marked languages, however, more information is often encoded syntactically.
Many languages, for instance, have an instrumental case, which explicitly marks the instrument of a sentence. However, in such languages the instrumental case may have other uses (such as being governed by certain prepositions).
Although either the patient or agent can function as the subject of a sentence even in unmarked usages, in ergative-absolutive languages the case marking of the "subject" differs depending on the type of verb used, in a way that tends to reflect the theta role it occupies.
Generally, only one noun phrase can occupy a certain theta role in a sentence. This does not include conjunctions such as Bill and Ted went shopping, or I was attacked by cats and by dogs. But a sentence such as The car broke the window with its fender strongly implies for most speakers that the car is acting as an agent, because the car and the fender cannot both be instruments.
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