A person feels morally free who feels that his habits or his temptations are not his masters, but he theirs
 John Stuart Mill, A System Of Logic, Ratiocinative And Inductive (1843). copy citation

Context

“but it is of great consequence that we should not be prevented from forming such a desire by thinking the attainment impracticable, and that if we have the desire, we should know that the work is not so irrevocably done as to be incapable of being altered. And indeed, if we examine closely, we shall find that this feeling, of our being able to modify our own character if we wish, is itself the feeling of moral freedom which we are conscious of. A person feels morally free who feels that his habits or his temptations are not his masters, but he theirs; who, even in yielding to them, knows that he could resist; that were he desirous of altogether throwing them off, there would not be required for that purpose a stronger desire than he knows himself to be capable of feeling.” source