I am here insensibly led to make an apology for the instability of the theories and practice of physic. The theory of physic is founded upon the laws of the animal economy. These (unlike the laws of the mind, or the common laws of matter) do not appear at once, but are gradually brought to light by the phænomena of diseases. The success of nature in curing the simple diseases of Saxony, laid the foundation for the ANIMA MEDICA of Doctor STAHL. The endemics of Holland[15] led Doctor BOERHAAVE to seek for the causes of all diseases in the FLUIDS. And the universal prevalence of diseases of the NERVES, in Great-Britain, led Doctor CULLEN to discover their peculiar laws, and to found a system upon them; a system, which will probably last till some new diseases are let loose upon the human species, which shall unfold other laws of the animal economy. [15] "The scurvy is very frequent in Holland; and draws its origin partly from their strong food, sea-fish, and smoked flesh, and partly from their dense and moist air, together with their bad water." Hoffman on Endemical Distempers. "We are now in North-Holland; and I have never seen, among so few people, so many infected with the leprosy as here. They say the reason is, because they eat so much fish." Howell's Familiar Letters. It is in consequence of this fluctuation in the principles and practice of physic, being so necessarily connected with the changes in the customs of civilized nations, that old and young physicians so often disagree in their opinions and practices. And it is by attending to the constant changes in these customs of civilized nations, that those physicians have generally become the most eminent, who have soonest emancipated themselves from the tyranny of the schools of physic; and have occasionally accommodated their principles and practice to the changes in diseases[16]. This variety in diseases, which is produced by the changes in the customs of civilized nations, will enable us to account for many of the contradictions which are to be found in authors of equal candour and abilities, who have written upon the materia medica.