It has often been noticed that one syntactic argument position can be realized by elements which seem to realize different thematic roles. This is notably the case with the external argument position of verbs of change of state which licenses volitional agents, instruments or natural forces/causers, showing the generality and abstractness of the external argument relation. (1) a. John broke the window (Agent) b. The hammer broke the window (Instrument) c. The storm broke the window (Causer) In order to capture this generality, Van Valin & Wilkins (1996) and Ramchand (2003) among others have proposed that the thematic role of the external argument position is in fact underspecified. The relevant notion is that of an effector (in Van Valin & Wilkins) or of an abstract causer/initiator (in Ramchand). In this paper we argue against a total underspecification of the external argument relation. While we agree that (1b) does not instantiate an instrument theta role in subject position, we argue that a complete underspecification of the external theta-position is not feasible, but that two types of external theta roles have to be distinguished, Agents and Causers. Our arguments are based on languages where Agents and Causers show morpho-syntactic independence (section 2.1) and the behavior of instrument subjects in English, Dutch, German and Greek (section 2.2 and 3). We show that instrument subjects are either Agent or Causer like. In section (4) we give an analysis how arguments realizing these thematic notions are introduced into syntax.