Our modern philosophers will not perhaps be able to understand, how it is that whilst in the greatest number of cases, the application of a degree of caloric superior or inferior to that of our temperature, is necessary to produce heat or cold, this sensation can arise in a part though it may not have experienced an increase or diminution of this principle. But in the greatest number of cases has not pain a material cause? And yet all sympathies produce it without this cause. The vulgar, who stop at the diversity of the modifications of feeling, believe that an insulated principle presides over each. Let us disregard all these modifications, in order to see but a single principle in the irregularities as in the regular course of sensibility. That this property, sympathetically altered, gives us the sensation of heat or cold as in the skin, of pulling as in the nerves, of lassitude as in the muscles in the beginning of a disease, &c.; these are but varieties of a single cause, one, of which we are ignorant, but which evidently exists. In general, the sympathies of animal sensibility put into action in each system the sensation which is usual there. The sympathy which, acting upon the skin, creates there a sensation of heat or of cold, would have produced that of lassitude if it had acted upon a muscle. In order to form an exact idea of heat and cold considered as sensations, let us recollect that they may arise from different causes. 1st. From the increase or diminution of the caloric of the atmosphere. 2d. From the disengagement or the want of disengagement of this fluid in a part of the economy, as in a phlegmon or after the ligature of an artery of a limb. 3d. Sometimes without previous inflammation more caloric is disengaged in the whole body; there is a general increase of temperature; we then feel an internal and external heat; or caloric is disengaged locally in a part of the skin, and the patient feels a heat there as he does who applies his hand upon this place. 4th. Finally, there are sympathies of heat and cold. Some other parts, besides the mucous surfaces and the skin, feel these sympathies; we know the sensation of coldness that is felt to arise from the abdomen to the thorax, &c.