There is but one soul throughout the universe, all is but one existence - "Thou art in the woman, thou in the man, thou in the young man walking in the pride of youth, thou in the old man tottering on his stick - thou art All - in all, in everything, and I am thee, because I am made from thee."
Just as the body has its progress and decadence, so also has the mind, and, therefore, the mind is not the soul, because the soul can neither decay nor degenerate.
Change is always subjective. All through evolution you find that the conquest of nature comes by change in the subject. Apply this to religion and morality, and you will find that the conquest of evil comes by the change in the subjective alone. That is how the Advaitic system gets its whole force, on the subjective side of man.
The hard discipline, with the exception of one great good point, is fraught with evil. The good point is that men can do one or two things well with very little effort, having practiced them every day through generations.
We want to lead mankind to the place where there is neither the Vedas, nor the Bible, nor the Koran; yet this has to be done by harmonizing the Vedas, the Bible, and the Koran.