The Spirits' Book
Book Two - The Spirit World
Chapter VIII - Emancipation of the Soul
448. Is second sight permanent?
“The ability is permanent, but its exercise is not. In less material worlds, spirits can free themselves more easily from matter and communicate with one another via thought, without excluding verbal speech. In those worlds, second sight is a constant ability for most of their inhabitants. Their normal state may be compared to that of waking somnambulism among you, and this is why they manifest themselves more easily to you than those who are incarnated in more rudimentary bodies.”