Some authors have considered diseases only as increased strength or weakness, and have consequently divided medicines into tonics and debilitants. This idea is true in part, but it is false when we generalize it too much. For every vital force there are means proper to raise it when too much diminished, and to lessen it when too much elevated. But tonics and debilitants are certainly not applicable to every case. You would not weaken animal contractility, augmented in convulsions, as you would insensible organic contractility increased in inflammation; neither would you increase them by the same means. The morbid phenomena that organic contractility and animal sensibility experience, are not cured by the same method. There are medicines proper for each vital force. Moreover, it is not only in increase or diminution that the vital forces err, but they are besides disordered; the different modifications that insensible contractility and organic sensibility can undergo, produce in wounds and ulcers a diversity of suppuration, in glands a diversity of secretion, in exhaling surfaces a diversity of exhalations, &c. It is necessary, therefore, that medicines should not only diminish or increase each of the vital forces, but that they should moreover restore them to the natural modification from which they have been altered. What I have just said is particularly applicable to the strictum and laxum of many authors, who every where see but these two things. The strictum may be properly applied to inflammation, the laxum to dropsies, &c.; but what have these two states of the organs in common with convulsions, with disorder of the intellect, with epilepsy, with bilious affections, &c.? It is the peculiarity of those who have a general theory in medicine, to endeavour to bend every phenomenon to it. The fault of generalizing too much, has been perhaps more injurious to science, than that of viewing each phenomenon separately. These observations are, I think, sufficient to show, that every where in the physiological sciences, in the physiology of vegetables and of animals, in pathology, in therapeutics, &c. there are vital laws, that govern the phenomena which are the object of these sciences; and that there is not one of these phenomena that does not flow from these essential and fundamental laws, as from its source. If I should take a survey of all the divisions of physical sciences, you would see that the physical laws were ultimately the sole principle of all their phenomena; but this is so well known that it is not necessary to do it. I will consider an important subject, and one to which we are naturally led by the preceding observations. I mean, a parallel between physical and vital phenomena, and consequently between physical and physiological sciences. III. _Characteristics of the vital properties, compared with those of the physical._