What can in fact be seen by the dead is spiritual knowledge, feeling and experience. That is the connection of souls here on earth with the spiritual world. If a soul who has been left behind on earth occupies himself with knowledge of the spiritual worlds, allows such thoughts to cross his mind, then these thoughts can be perceived by the one who has died.
Source: Rudolf Steiner – GA 140 – Life Between Death and Rebirth – X – Tübingen, February 16, 1913
Translated from shorthand reports unrevised by the lecturer, by R. M. Querido.