structures of the body, and even in the same individual at different times under different circumstances. The strength of the “excitement” depends on the one hand upon the degree of irritability, and on the other upon the strength of the stimulus. The irritability itself is influenced and changed by the action of the stimuli. If the stimuli are too strong and are of prolonged duration, the irritability diminishes as a result of exhaustion; if weak stimuli act during a prolonged time, the irritability increases. The healthy organism has a mean degree of irritability. Disease occurs when this state is altered by _strong_ stimuli or by an _absence_ of stimulation. Disease and health, therefore, differ not qualitatively but only quantitatively. It is here seen that we have the first attempt at a systematic interpretation of the effects of stimulation, and it is astonishing how sharply and successfully _Brown_ has pointed out the foundations of this important field. He has in this way not only amply compensated for the great setback in the history of the teaching of irritability produced by the confusions of conceptions created by _Haller_ and the vitalists, but also placed the whole of the physiology of stimulation on a firm foundation upon which it is possible to build further. Though it is true that many of his special theories, in particular those on nature and the origin of disease, are quite erroneous, still a just critic must judge work in relation to the period in which it was written, and I question if at the present day the science of medicine does _not_ contain teachings which in a hundred years will also prove untenable. [4] _John Brown_: “Elementa medicinæ.” 1778. English translation. London 1778.