“Q: What is the condition of the Linga Sarira, or plastic body, during dreams?
A: The condition of the Plastic form is to sleep with its body, unless projected by some powerful desire generated in the higher Manas.
In dreams it plays no active part, but on the contrary is entirely passive, being the involuntary half-sleepy witness of the experiences through which the higher principles are passing.
Q: Under what circumstances is this wraith seen?
A: Sometimes, in cases of illness or very strong passion on the part of the person seen or the person who sees; the possibility is mutual.
A sick person especially before death, is very likely to see in a dream, or vision, those whom he loves and is continually thinking of, and so also is a person awake, but intensely thinking of a person who is asleep at the time.
Q: Can a magician summon such a dreaming entity and have intercourse with it?
A: In black Magic it is no rare thing to evoke the “spirit” of a sleeping person; the sorcerer may then learn from the apparition any secret he chooses, and the sleeper be quite ignorant of what is occurring. Under such circumstances that which appears is the Mayavi rupa; but there is always a danger that the memory of the living man will preserve the recollections of the evocation and remember it as a vivid dream.
If it is not, however, at a great distance, the Double or Linga Sarira may be evoked, but this can neither speak nor give information, and there is always the possibility of the sleeper being killed through this forced separation. Many sudden deaths in sleep have occurred, and the world would be no wiser.”
H. P. Blavatsky
The Mark Of God: Understanding Your "I AM Presence" 