The Ringwraiths -- What were they?

I still think that the nazgul were remnants, or memories of the men that were bound to the dark lord through their rings. The power of the ring is not really earthly, so after they became consumed by their greed and will to possess the rings, they weren't "earthly" either; more like spirits of their inner self. Does that make sense? They weren't the bodies anymore, but the spirits inside themselves that became enslaved through the rings.