The first attempt to give _Glisson’s_ expression “irritability” a more concrete meaning was made by _Haller_ (1708–1777)[3]. Unfortunately, though, he confined this conception solely to muscles, in that he understood by the term irritability “the capability of the muscles to contract, when stimulated, as the result of vital force (_vi viva_).” He, therefore, applied the term “irritability” to that which we today refer to as “contractility.” On the other hand he applied the term contractility solely to a property possessed by other living and dead animal as well as vegetable matter, elasticity, that is, the capability to resume its original form after distortion. He makes a sharp distinction between “irritability,” which manifests itself by a contraction of the muscles after stimulation by its own vital force (_vi viva_), and the “sensitivity,” which is possessed only by the nervous system. “_Sola fibra muscularis contrahitur vi viva; sentit solus nervus et quæ nervos acciperunt animales partes._” By confining the conception of irritability to a single living substance, the muscle, _Haller’s_ theory represents a great regression in comparison to the correct fundamental thoughts of _Glisson_. This unfortunate use of the term of “irritability,” “contractility” and “sensitivity” has opened wide the gates to confusion and misunderstanding. This confusion was still further augmented by the fact that the vitalistic school of Montpelier confused the idea of vital force with that of irritability. In the works of _Bordeu_ (1722–1776) these views are comparatively clear, if one bears in mind that he substitutes _Glisson’s_ term of “_irritability_” with that of “_sensitivity_.” He assumes a “_sensibilité générale_” or a common property of all living structures, both solid and fluid. Besides this, each different part has according to him its “_sensibilité propre_.” Here in place of the clear conception of irritability we find one of more or less mythical nature possessing traces of _Stahl’s_ “anima.” Nevertheless we observe here the idea that all living organisms possess in common a capability to respond to stimuli. Even though _Bordeu’s_ differentiation of the “sensibilité propre” and the “sensibilité générale” is too artificial and the coexistence of both not justifiable, his discussion of the “sensibilité propre” shows that he is already on the track of the characteristics of the effect of stimuli which only later under the name of “specific energy” was clearly recognized as a fundamental property of all living substance. On the other hand the celebrated pupil of _Bordeu_, _Barthez_ (1734–1806), accepted the existence of a meaningless vital principle, the “_principe vitale_,” governing all vital manifestations. The two forms of vital force of all living substances, the “_forces sensitives_” and the “_forces motrices_,” were according to his views manifestations of this vital principle. He differentiates the “_force sensitive_” into a “_sensibilité avec