In certain theories of linguistics, thematic relations, also known as semantic roles, are the various roles that a noun phrase may play with respect to the action or state described by a governing verb, commonly the sentence's main verb. For example, in the sentence "Susan ate an apple", Susan is the doer of the eating, so she is an agent; an apple is the item that is eaten, so it is a patient.
Since their introduction in the mid 1960s by Jeffrey Gruber and Charles Fillmore, semantic roles have been a core linguistic concept and ground of debate between linguist approaches, because of their potential in explaining the relationship between syntax and semantics (also known as the syntax-semantics interface), that is how meaning affects the surface syntactic codification of language. The notion of semantic roles play a central role especially in functionalist and language-comparative (typological) theories of language and grammar.
While most modern linguistic theories make reference to such relations in one form or another, the general term, as well as the terms for specific relations, varies: "participant role", "semantic role", and "deep case" have also been employed with similar sense.
The notion of semantic roles was introduced into theoretical linguistics in the 1960s, by Jeffrey Gruber and Charles Fillmore, and also Jackendoff did some early work on it in 1972.
The focus of these studies on semantic aspects, and how they affect syntax, was part of a shift away from Chomsky's syntactic-centered approach, and in particular the notion of the autonomy of syntax, and his recent Aspects of the Theory of Syntax (1965).
The following major thematic relations have been identified:
deliberately performs the action (e.g. Bill ate his soup quietly). The actions can be both conscious or unconscious. In syntax, the agent is the argument of a transitive verb that corresponds to the subject in English.
the entity that receives sensory or emotional input (e.g. Susan heard the song. I cried).