" is not in control to do the opposite when he so desires," even when he himself wants something else now- he's incapable. why? "for his will is bound with habit, which becomes second nature to him" he literally compares it to a prison, this person is in prison-because his will, which does want to do the right thing- and in his opinion is the correct and proper thing to do, and must be done- but that will is imprisoned.