(Material aspects)
Dreaming of visiting a medium very often means that we are looking for some kind of contact with our own unconscious, or with the dead. We may also be attempting to alert our own intuition and use it differently to a way we have done previously.
At a very basic level, we all have fears and feelings about death, and the actual appearance in dreams of spiritual entities can help us to come to terms with these. You might like to consult the entry for dead people.
