Thematic Hierarchies
Drawing on observations based in typological cross-linguistic comparisons of languages (Fillmore 1968), linguists in the relational grammar (RG) tradition (e.g. (Perlmutter & Postal 1984) observed that particular thematic relations and theta roles map on to particular positions in the sentence. For example, in unmarked situations agents map to subject positions, themes onto object position, and goals onto indirect objects. In RG, this is encoded in the Universal Alignment Hypothesis (or UAH), where the thematic relations are mapped directly into argument position based on the following hierarchy: Agent < Theme < Experiencer < Others. Mark Baker adopted this idea into GB theory in the form of the Uniformity of Theta Assignment Hypothesis (or UTAH) (Baker 1988). A different approach to the correspondence is given in (Hale & Keyser 1993) and (Hale & Keyser 2001), where there are no such things as underlying theta roles or even thematic relations. Instead, the interpretive component of the grammar identifies the semantic role of an argument based on its position in the tree.
Read more about this topic: Theta Role