"The nature of the fluidic covering always relates directly to the degree of the spirit's moral advancement.  The inferior spirits can not change their covering as they wish, just as they can not pass, at their own will, from one world to another.  There are some, however, whose fluidic covering, despite being ethereal and imponderable in reference to tangible matter, is still so heavy with reference to the spiritual world, that it does not allow the spirit to leave its own environment.  In this category, we include those spirits whose perispirit is so dense, that they confuse it with the carnal body and continue to believe that they are still alive. These spirits, large in number, stay on the Earth's surface, like the incarnate, believing themselves to be still involved in their Earthly occupations and activities.  Others, slightly less materialized, are not so enough to be able to rise above the Earthly regions. 
         On the contrary, the superior spirits can come to the inferior worlds, and even incarnate in them. They take, from the constructive elements of the world they have entered, the materials for the formation of the fluidic or corporeal body, appropriate for the environment of that world. They do this in the same way that a noble man changes, temporarily from his usual, grandiose attire to the clothing of a commoner, all the while remaining a noble man.
         This is how the spirits of the most elevated categories are able to manifest themselves to Earth's inhabitants, or to even incarnate among them as part of a mission. Such spirits bring with them not the covering, but an intuitive memory of the regions in which they lived and which, in thought, they are able to see. They are seers among the blind."