analyzed by the explanation that we have the “_Ekphorie_” of a state of excitation that was once previously produced by an original stimulus of some unknown kind, and of which the living substance of the muscle, in consequence of its “_Mneme_,” has retained a latent “_Engramm_”? Here the deep gulf is apparent which exists between the demands of a physiological analysis and the futile explanation of the mneme doctrine. Physiological investigation must reject such a manner of treating its problems. [11] _Semon_: “Die Mneme als erhaltendes Princip im Wechsel des organischen Geschehens.” Zweite verbesserte Auflage, Leipzig. [12] _Ewald Hering_: “Uber das Gedächtniss als allgemeine Function der organischen Materie.” Wein 1876. [13] _Ernst Haeckel_: “Die Perigenesis der Plastidule oder die Wellenzeugung der Lebenstheilchen.” Berlin 1876. With this the history of the doctrine of irritability enters into its present phase of development. To future research remains then the problem of further analyzing irritability, this common property of living substance, and finally rendering it into its simplest chemical and physical components. This last goal can only be approached very gradually, step by step. With the analysis of irritability we shall investigate life itself. In the following lectures it will be my endeavor to show how far, with our present knowledge, we can penetrate by this path into the great secret. CHAPTER II THE NATURE OF STIMULATION _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. The common problem of all scientific research is the investigation and formulation of natural laws. The assumption of a unity in the happenings and of existence in the world, in accordance with definite laws, forms the indispensable foundation of all scientific study and is fully justified by experience. Experience has taught us, as a result of innumerable individual observations, the existence of such an accordance, whereas in not a single instance has it been shown that this is not the case. We are thus justified in assuming without further discussion that every scientific research, every new problem which we approach, is likewise founded on this unity of occurrences in accordance with natural laws. Only on the firm basis of this assumption has scientific investigation a purpose, and every success is a new proof of this. There is an unanimity of opinion concerning this among scientific investigators in all fields.