Stahl, less brilliant than profound, rich in the means that convince, though deficient in those that please, formed for the physiological sciences an epoch more worthy of notice than that of Boerhaave. He perceived the discordance between the physical laws and the functions of animals; this was the first step towards the discovery of the vital laws, but he did not discover them. The soul was to him every thing in the phenomena of life; it was much to neglect attraction and impulse. Stahl perceived that these were not true, but the truth escaped him. Many authors, following his steps, have referred to a single principle, differently denominated by each, all the vital phenomena. This, called the vital principle by Barthez, archeus by Van Helmont, &c. is a speculation that has no more reality than that which would refer to a single principle all the physical phenomena. Among these we know that some are derived from gravity, some from elasticity, others from affinity, &c. The same in the living economy, some are derived from sensibility, others from contractility. Unknown to the ancients, the laws of life have begun to be understood during the last age only. Stahl had already remarked the tonic motions, but he did not generalize their influence. Haller was engaged particularly with sensibility and irritability; but in limiting one to the nervous system, and the other to the muscular, this great man did not consider them in the correct point of view; he made them almost insulated properties. Vicq d'Azyr changed them into functions in his physiological division and ranked them with ossification, digestion, &c. that is, he confounded the principle with the consequence. Thus you see, notwithstanding the labours of a crowd of learned men, how much the physiological sciences still differ from the physical. In these, the chemist refers all the phenomena that he observes to affinity: the natural philosopher, in his science, every where sees gravity, elasticity, &c. In the others, we have not as yet ascended, at least in a general manner, from the phenomena to the properties from which they are derived. Digestion, circulation, or the sensations, do not bring the idea of sensibility or contractility to the mind of the physiologist, as the movement of a watch proves to the mechanician that elasticity is the primum mobile of its motion; or as the wheel of a mill or of any machine, which running water sets in motion, proves to the natural philosopher that gravity is the cause. To place upon the same level in this respect these two classes of sciences, it is evidently necessary to form a just idea of vital properties. If their limits are not accurately assigned, we cannot with precision analyze their influence. I shall present here only general considerations on this point, which has been treated sufficiently in my Researches upon Life; what I shall add now will be but as a supplement to what has been explained in that work.