  The way out of the puzzle is to see that the relation between an agent and the event he or she efficiently causes is not the same as the relation between the agent and her exertion of power. Trying, on this view, is not something that an agent does in order to do something else; it is not a separate action on the way to performing another act. If I am going to build a birdhouse, I first have to buy nails. Buying nails is a distinct act from building the birdhouse that must be done on the way to building the birdhouse. But trying to buy the nails is not distinct from buying them in the way that buying them is distinct from building the birdhouse. If I manage to buy the nails, I haven't done two things, buy them and try to, but only one. But if I manage to build the birdhouse, then I did do two things: buy the nails, and build the birdhouse. So, there is something wrongheaded about even asking whether the agent is the efficient cause of the exertion of power. The agent is the efficient cause of the act, and there is no separate act, the exertion, of which he or she needs to be the efficient cause in order to have acted freely.