How is it that primitive man, provided with five senses which bring him into contact with the material world only, has found it possible to conceive the existence of an invisible world peopled with beings whom his eyes cannot see, nor his hands touch, nor his ears hear? Between the birth of human reason and the invention of writing a long period of time elapsed; when the art of writing was followed by that of printing, man then printed all that he had thought and written, and at present we possess thousands of volumes which will inform us on all the truths and errors which have alternately illuminated and obscured the human mind. Whoever would take the trouble to examine this mass of documents, and read those which furnish an approximate estimate of the mental activity of our primitive ancestors, will see that the human _ego_ pursued science unconsciously long before scholars appeared, and applied the name of philosophers to themselves, because they had sought patiently and with many discussions, through thousands of centuries, to find the best way of arriving at the truth. These ancestors of ours were of an enquiring turn of mind. The appearance of religion amongst men is at the same time the most natural and the most supernatural fact in the history of humanity. The greater number of philosophers have recognised that the tendency of the human mind to turn towards that which is outside the domain of the senses is as powerful in man as the desire of eating and drinking is in all living beings. The ancients acknowledged this to be a true sense, as irresistible as the rest of the operations of our external senses, and they have well named it _sensus numinis_--the consciousness of the divine. The desire of understanding the secrets with which the Unknown was invested naturally led to the investigation of the influence which these secrets might exercise on the destinies of mankind. Amongst certain peoples this gave birth to the art of divination. To this they abandoned themselves in all sincerity, not doubting that omnipotent beings would always be ready to make their will known to mortals.