In order for us to understand the physical nature of spirits, the spirit realm, and the interaction between the material and spiritual worlds, it is imperative that we have an understanding of some basic concepts regarding spirit and matter.   Some of these concepts were, in part, proposed by scientists and philosophers even before the codification of Spiritism.  The revelations of the superior spirits, then, confirmed and/or clarified elements of the scientists’ theories and provided added explanation. These revelations teach us about the two general elements of the universe  ̶  matter and spirit  ̶  and their intermediary element, the cosmic principle.  Upon these elementary concepts, we are able to build our knowledge about our own nature and that of the world(s) we inhabit.