Let's begin with a sentence: "Mark hit the ball with the bat." What is the verb? Hit, right? There are other elements of this sentence, too, and they all relate in some way to the verb “hit.” “Mark” is the one doing the action. The “ball” is the thing that receives the action. The “bat” is the instrument used to perform the action. Theta theory is an area of linguistics that examines the relationships between a verb like “hit” and all the other elements of a sentence. These elements, if they are necessary to the meaning of the sentence, are called “arguments.” So “Mark,” “ball,” and “bat” are the arguments of the verb “hit.”
Theta theory assigns a particular role to each argument in a sentence according to how it relates to the verb. Linguists identity these “theta roles” or “thematic roles” in various ways, but the following are most common:
- An agent performs an action. In our example sentence, Mark is an agent; he performs the action of hitting the ball.
- A theme receives the action. The ball is the theme.
- An instrument is used to carry out the action. The bat is the instrument.
- A goal is the end point toward which the action moves. If we had said, “Mark hit the ball toward the fence,” we could label “fence” as the goal.
- A source is the origin of the action. We might say, “Mark hit the ball from home plate,” and “home plate” would be the source.
- A location is where an action occurs. If we say, “Jennifer caught the ball at first base,” “first base” is the location where the action happens.
- A patient is something that is changed by an action. In the sentence “John burned his hamburger,” “hamburger” is the patient, for it has undergone a change. It was not previously burned, but now it is.
- An experiencer experiences some sort of emotion or perception or state of being expressed by a verb. If we say, “Amber enjoys hiking,” we can label Amber as an experiencer because she is experiencing the emotion of enjoying something.
- A benefactive receives something (positive or negative) by the verb's action. In the sentence, “John hit Joe with his famous one-two punch,” Joe is the benefactive of the action.
These roles can be quite slippery, and even experienced linguists have difficulty identifying them accurately in some sentences. For example, some linguists might call “Joe” in the sentence above the theme or even the goal of the verb rather than the benefactive. We know, however, thanks to the “theta criterion” that every argument can have only one theta role and that no theta role can be repeated in any sentence.
Linguists also create “theta grids” to show how each argument in a sentence is related to the verb. For the sentence “Mart hit the ball with the bat,” the theta grid would look like this:
hit <agent, theme, instrument>
For the sentence, “Jennifer caught the ball at first base,” we would create the theta grid as follows:
caught <agent, theme, location>
Please remember that theta theory is complex. The above discussion is merely a brief and limited introduction, but it does provide an overview of a linguistic method for describing the relationships between a verb and the rest of the sentence.