Halliday & Matthiessen (1999: 517):
When it comes to mental processes (and here is one of the contradictions referred to above), the grammar is uncertain whether the participant other than the Senser is doing duty as agent or not; if I'm doubtful about something, for example, I may say your story doesn't convince me, which makes your story look like an agent, or I may say I don't believe your story, which makes the role of your story very different — not exactly a goal, but like an expression of scope. Many languages display some such ambivalence about mental processes, which do not match up neatly with material processes in our experience in any obvious way.