Sinistrari is careful to explain his terms at the outset, which is helpful because his treatment of the Incubi and Succubi are not conceived in the ordinary acceptance of the words. The thesis which Sinistrari sets forth may be partially explained thus: there are in existence on earth rational creatures besides man, endowed like him with a body and a soul and are capable of being saved or being lost. These creatures are affected with the same emotions as man, and they are able to withdraw from the sight of man at will.