There are, says Cutten, three fairly well defined periods in the history of Medicine. The first, beginning as far back as anything human begins and coming down to the end of the second century; the second, ending with the fifteenth or sixteenth centuries; and the third from perhaps the sixteenth century on. The second period, he adds, was by far the most sterile and stationary of the three "largely due to the prohibitive attitude of the Church. The science of Medicine, then, is almost wholly the result of the investigations and study of the last period. This means that medicine is one of the youngest of the sciences, while from the very nature of the case it is one of the oldest of the arts." Demon possession was, as has been said, the earliest explanation of disease. This would naturally be true of a time almost wholly wanting in any conception either of law or any relation of cause and effect beyond the most limited regions of experience. Since the only cause of which man had any real knowledge was his own effort he peopled his world with forces more or less like himself, except that they were invisible, who operated practically the whole of natural phenomena. There was a spirit for every place and every happening; spirits for fields and hearths, thresholds and springs. Some of them were friendly, some of them naturally unfriendly, but they were everywhere in existence, everywhere in action and naturally if they were unfriendly they would from time to time and in various most curious ways get into the body itself and there do any amount of mischief. The priest-doctor's task, therefore, was to get them out. He might scare them out, or scold them out, or pray them out, or trick them out. He would use his medicine as much to make the place of their temporary abode uncomfortable for the demon as remedial for the patient and, indeed, the curious and loathsome things which have been used for medicines might well disgust even a malevolent demon. One thing stands out very clearly and that is that whatever the medicine did or left undone, it worked through its influence upon the mind of the patient and not through any real medicinal value. _The Beginnings of Scientific Medicine_