However, thematic roles also introduce new difficulties. As pointed out by Dowty (1991), thematic roles lack well-defined semantics. For example, while (3.1) clearly involves an animate agent acting causally upon a physical object, and the PP evidently supplies a goal location, it is much less clear what the roles should be in (web-derived) sentences such as (3.2–3.4), and what semantic content they would carry: