In our example sentence, one role is taken by the noun phrase The boy as “the entity that performs the action,” technically known as the agent. Another role is taken by the ball as “the entity that is involved in or affected by the action,” which is called the theme (or sometimes the “patient”). The theme can also be an entity (The ball) that is simply being described (i.e. not performing an action), as in The ball was red.

Agents and themes are the most common semantic roles. Although agents are typically human (The boy), they can also be non-human entities that cause actions, as in noun phrases denoting a natural force (The wind), a machine (A car), or a creature (The dog), all of which affect the ball as theme.

The theme is typically non-human, but can be human (the boy), as in The dog chased the boy. In fact, the same physical entity can appear in two different semantic roles in a sentence, as in The boy cut himself. Here The boy is agent and himself is theme.