His first appearance in literature was the publication of his _New Theory of Fevers_, written in defence and at the suggestion of his old master Dr. Pitcairn, who was at war with his brethren on the nature of epidemics. The author, while in after life holding that it contained, though in a crude form, some valuable matter, wisely allowed it to fall into oblivion. The Mechanical or _Iatro-Mathematical_ Theory, as it was called, of which Cheyne was one of the earliest and most distinguished expounders, by which it was attempted to apply the laws of Mechanics to vital phenomena, had succeeded to the principles of the old Chemical School. On the Continent the new theory had the support of the eminent authority of Boerhaave, Borelli, Sauvages, Hoffman, and others. The natural desire to discover some definite and simple _formulæ_ of medical science lay at the root of this, as of many other hypotheses. Cheyne, himself, it is right to observe, ridiculed the notion that all vital processes can be explained on mechanical principles. In 1705 he published his _Philosophical Principles of Natural Religion_, a book which had some repute in its day, apparently, since it was in use in the Universities. Between this and his next essay in literature a long interval elapsed, during which he had to pay the penalty of his old habits in apoplectic giddiness, violent headaches, and depression of spirits. Happily, it became for him the turning-point in his life, and eventually rendered him so useful an instructor of his kind. He had now arrived at a considerable amount of reputation in the profession. He seems to have been naturally of agreeable manners and of an amiable disposition, as well as of lively wit which, improved by study and reading, made him highly popular; and amongst his scientific and professional friends he was in great esteem. He had now, however--not too soon--determined to abandon his _bon-vivantism_, and speedily “even those who had shared the best part of my profusions,” he tells us, “who, in their necessities had been relieved by my false generosity, and, in their disorders, been relieved by my care, did now entirely relinquish and abandon me.” He retired into solitude in the country and, almost momentarily expecting the termination of his life, set himself to serious and earnest reflection on the follies and vices of ordinary living.