_Contents_: Introductory. Earliest period. _Francis Glisson_ as founder of the doctrine of irritability. _Albrecht von Haller._ The vitalists. _Bordeu_ and _Barthez_. _John Brown’s_ system. _Johannes Müller_ and the specific energy of living substance. _Rudolf Virchow’s_ doctrine of the irritability of the cell. Discovery of the inhibitory effects of stimulation. _Weber_, _Schiff_, _Goltz_, _Setschenow_, _Sherrington_. _Claude Bernard_ studies on narcosis. Tropisms. _Ehrenberg_, _Engelmann_, _Pfeffer_, _Strassburger_, _Stahl_. _Semon’s_ speculations on mneme. 1 II _Contents_: Principles of scientific knowledge and research. Origin and meaning of the conception of cause. Cause and condition. Criticism of the conception of cause. The conditional point of view. Conception of cause. The conditional point of view applied to the investigation of life. Conception of vital conditions. Definition of the conception of stimulation. 18 III _Contents_: The quality of the stimulus. Positive and negative alterations of the factors which act as vital conditions. Extent of the alteration in vital conditions or intensity of the stimulus. Threshold stimuli, sub-threshold, submaximal, maximal and supermaximal intensities of stimulus. Relations between the intensity of stimulus and the amount of response. The _Weber_ and _Fechner_ law. All or none law. Time relations of the course of the stimulus. Form of individual stimulus. Absolute and relative rapidity in the course of the stimulus. Duration of the stimulus after reaching its highest point. Adaptation to persistent stimuli. Series of individual stimuli. Rhythmical stimuli. The _Nernst_ law. 39 IV _Contents_: Various examples of the effects of stimulation. Metabolism of rest and metabolism of stimulation. Metabolic equilibrium, Disturbances of equilibrium by stimuli. Quantitative and qualitative alterations of the metabolism of rest under the influence of stimuli. Excitation and depression. Specific energy of living substance. Qualitative alterations of the specific metabolism and their relations to pathology. Functional and cytoplastic stimuli. Relations of the cytoplastic effects of stimuli to the functional. Hypertrophy of activity and atrophy of inactivity. Metabolic alterations during growth of the cell. Primary and secondary effects of stimulation. Scheme of effects of stimulation. 65 V