: Now, the question is, what happens when someone dons a Ring of Power which renders them "invisible". Invisibility is simply a state of existing while not being seen. Being invisible doesn't mean one is intangible. But in fact if one remained invisible too long, one would fade and become a wraith. A wraith seems to be only the will and spirit-stuff, without the body.

I would agree that a wraith is "spirit-stuff" without the body but definately NOT the will. The Nazgul were controlled by the will of Sauron and did not have a will of their own. When Sauron was "destroyed" the Nazgul dissipated as their was no will to control them. When Frodo put on the Ring he could not help being controlled by the Sauron's will.

Therefore, I agree that all being are composed of a body (at least mortal beings), "spirit-stuff" and will, but wraiths would be only spirit-stuff without bodies and their wills would be that of whoever was controlling them ie. Sauron.