PID and Moral Responsibility

PAP, otherwise known as the principle of alternative possibilities, is the claim that someone is not morally responsible for actions when he could not have done otherwise.  How we define a person certainly has consequences, then, for claims about personal moral responsibility.  Recall that Locke thinks that humans and animals are different than machines because whereas machines must be operated by someone, humans and other animals are the source of their own movement. If determinism is true, then it seems like even our own thoughts are mechanical insofar as they are caused by external forces.  So if humans are 'mechanical' in this sense, then they never seem to have the ability to act other than they have.  As a result, it seems like humans could never be morally responsible.