perception_” and “_sensibilité sans perception_,” using the term sensibility in the sense adopted by _Bordeu_ and which today we, with _Glisson_, call irritability. [3] _Albrecht v. Haller_: “Elementa Physiologiæ corporis humani.” Tomus IV. Lausannæ M D C L XVI. In this way serious thinkers of that time trifled with the words irritability, sensitivity, contractility, perception. This led to futile conceptions, which equalled the phantasies of the worst period of speculative philosophy and which in no way led to progress. Hence it is easy to understand that numerous attempts were made in those days to reconcile in some way these different conceptions. An explanation, which was the beginning of further development, came from England in the works of _John Brown_ (1735–1788),[4] a man who was as talented as he was dissolute. _Brown_ was an independent thinker, not without genius, whose knowledge in practice and theory, however, was limited. This combination in his mentality enabled him to observe the problems somewhat differently than through the glasses of the usual conceptions of that time. In direct opposition to his teacher _Cullen_ (1712–1790), one of the leading minds in the medical school of Edinburgh, who considered irritability only as an effect of sensibility and pronounced the latter a specific property of the nervous system, _Brown_ took the standpoint that all living substance, vegetable as well as animal, in contrast to lifeless matter, possessed a fundamental property which he designated as excitability, that is to say, the capability of being stimulated to specific vital manifestations through external factors or “stimuli,” in which sensitivity and indeed all mental processes as well as movement are interpreted as specific effects, which the “stimuli” produce on the irritable organs. This was an important advance and from a wilderness of trifling conceptions his observations led to a clearer knowledge of this subject. But _Brown_ went even further. In his so-called “theory of irritation,” he has presented a whole system of responsivity to stimulation, which in the first chapters of his chief work he expounds with wonderful clearness. The fundamental principles here established must be accepted even today. The essential basis of this “theory of irritability” which he worked out especially for his doctrine of disease, and which has also played an important part in pathology, is the following: Every living, that is, excitable system, is continually influenced by stimuli. The stimuli consist of either external factors, such as heat, food, foreign matter, poisons, etc., or inner factors which result from the influence of the activity of one organ upon another. Only as a result of the continual action of stimuli is life maintained, in that the stimuli produce continual “excitement” in the irritable substance. The degree of irritability differs in various plants, animals, in different