I am in due receipt of your letter dated September 19, 1975 and have noted the contents. I am very glad that you are seriously trying to understand our Krishna consciousness philosophy. It is Srila Jiva Goswami who has presented very high philosophy in his Six Sandavas.
Regarding your question about the difference between the mind and the soul, in the Bhagavad-gita it is directly said that the mind is inferior energy in a subtle form, and soul, jiva, is superior energy. So they are completely distinct. Mind is not spiritual, but mind is a subtle material form. When the soul becomes captivated for enjoying the material world instead of rendering service to Krishna, that is the beginning of his falldown. When the living being thinks himself to be the enjoyer, that is called false ego. His constitutional position is to serve Krishna. So this false egotism degrades him to pollute the intelligence and the mind.