25. The communications between the visible and invisible worlds may be secretive or open, spontaneous or induced.
Spirits act upon humans secretively through the thoughts they suggest to them and through certain influences; they act openly through effects discernible to the senses.
Spontaneous manifestations occur unexpectedly and fortuitously. Quite often, they occur to persons unfamiliar with Spiritist ideas, and who, for that very reason, cannot understand them; consequently, they attribute them to supernatural causes. Manifestations that are induced occur by means of certain individuals endowed with special faculties for producing such effects, and who are designated by the name mediums.