_Johannes Müller_ (1801–1858) then added an important stone to the building up of our knowledge of irritability. This was the clear recognition of the _specific energy_ of living substances. We have already found the germ in _Bordeu’s_ term “_sensibilité propre_” or “_sensibilité particulière_.” _Brown_ was also of the opinion that different living objects possessed different types of irritability and that excitation of their special functions was not dependent upon the _kind_ of stimulus acting upon them. _Johannes Müller_, grasping the idea hidden in this presentation, transformed it into a clear and fundamental conception. Already in the work written in his early years treating of optical illusions he says:[5] “It is immaterial by which means the muscle is stimulated, whether by galvanism, chemical agents, mechanical irritation, inner organic stimuli or sympathetic response from quite different organs; to every means by which it is stimulated and an effect produced, it responds by movement. Movement is, therefore, the _effect_ and the _energy_ of the muscle at the same time.” “Thus it is throughout with all reactions in the organisms.” “The sensory nerve, responding to any stimulus of whatever kind, has its specific energy; pressure, friction, galvanism and inner organic stimuli produce in nerves of sight that which is peculiar to them, light sensation; in the nerves of hearing, that which is peculiar to them, sound sensation; and in the nerves of touch, touch sensations. On the other hand, everything which affects a secretory organ produces change of the secretion; that which affects the muscle, movement. Galvanism is not superior to any other methods, of whatever kind, which can bring about stimulation.” And in his handbook of physiology _Johannes Müller_[6] formulates the law of specific energy for the sensory structures briefly in the following words: “The same external factor produces different sensations in the different senses according to the nature of each sense, namely, the sensation of the particular sensory nerves; and the reverse: the characteristic sensations peculiar to every sensory nerve can be produced by several internal and external influences.” This doctrine of the specific energy of the sense substance possesses an importance which extends far beyond the domain of the physiology of stimulation, for it forms the basis on which the whole theory of human knowledge must be built up, no matter how it may be constructed in detail. [5] _Johannes Müller_: “Über die phantastischen Gesichtserscheinungen. Eine physiologische Untersuchung mit einer physiologischen Urkunde des Aristotles über den Traum, den Physiologen und den Arzten gewidmet.” Coblenz 1826. [6] _Johannes Müller_: “Handbuch der Physiologie des Menschen für Vorlesungen.” Coblenz 1837.