The subject John in (58a) is both agent and theme: it is agent since it initiates and sustains the movement but also theme since it is the object that moves.35 Also, the subject Mary in (58b) can either be an experiencer or an agent depending on her intention one can just look at the 36 sky with no purpose at all.
Even though there are theoretical issues involved in adopting semantic roles in the grammar, there are also many advantages of using them. We can make generalizations about the grammar of the language: typically the 'agent' takes the subject position, while an NP following the word from is serving as the "source As we will see in the next chapter, semantic roles are also recognized as the standard concepts used for organizing predicate-argument structures for predicates within the lexicon. In the subsequent chapters, we will refer to semantic roles in various places.