_Cogito ergo sum_, “I think, therefore I am,” is a maxim of Descartes. What we think and give shape to in thought has for us a real existence, and we have it in our power to create thoughts that will have either a painful or pleasurable sensation. Painful sensations have occurred to persons by the conviction of the existence of a cause which would, when present, have produced certain results. Of this several examples are given in W. B. Carpenter’s physiology: “A clergyman told me that some time ago suspicions were entertained in his parish of a woman who was supposed to have poisoned her newly-born infant. The coffin was exhumed, and the coroner, who attended with the medical men to examine the body, declared that he already perceived the oder of decomposition, which made him feel faint, and in consequence he withdrew. But on opening the coffin it was found to be empty, and it was afterwards ascertained that no child had been born, and consequently no murder committed.” The second case is yet more remarkable: “A butcher was brought into the drug store of Mr. Macfarlan, from the market-place opposite, laboring under a terrible accident. The man, on trying to hook up a heavy piece of meat above his head, slipped, and the sharp hook penetrated his arm, so that he himself was suspended. On being examined he was pale, almost pulseless, and expressed himself as suffering acute agony. The arm could not be moved without causing excessive pain, and in cutting off the sleeve he frequently cried out. Yet when the arm was exposed, it was found to be quite uninjured, the hook having only traversed the sleeve of his coat.” In this, and similar cases, the sensation was perfectly real to the individual who experienced it, but it originated in the mind by an impression through the nerves of _internal sensation_ which created the idea or image in the brain, and the _external senses_ to which it was referred had nothing to do in causing the feeling. Diseases are thus created every day, either by ourselves or by those to whom we go for advice. I call to mind a lady who had gone to a distinguished practitioner for a supposed womb disease for some six months. She experienced no change for the better, but kept on growing continually worse, so that she no longer had a refreshing sleep, and her appetite for food was entirely gone. On examination I found her womb entirely healthy, in fact, exceptionally so, thanks to her attending physician, because, after a certain amount of useless doctoring, the rule is quite the other way. I told this lady of her error or delusion respecting her womb, and prescribed a quieting mixture for the night and a tonic for the day. She began at once to improve, and when I saw her again, six weeks afterwards, she had so fleshed up that I failed to recognize in her traces of her former delusion. The disease of which this woman was suffering was imaginary, and had no real existence for anyone outside of herself.