In presenting the conception of stimulation from a conditional standpoint, I desired to show what difficulties stand in the way of a theoretical isolation of a fundamental conception in the field of physiology, which indeed is used in our practical research work at every step. “_Natura non facit saltus._” I wished to demonstrate that the sharp separation of the conception of stimulation, like all artificial divisions which we make in nature, must always contain an arbitrary note, as in reality isolated systems do not exist in the world. I wished to show that, for this reason, the conception of vital system, the conception of life, the conception of vital conditions are not sharply defined. I wished likewise to show that as a necessary consequence of this fact a sharp separation of the conception of stimulation, which can only be made in relation to that of vital conditions, cannot be maintained theoretically. I wished to show further that there is no sharp line of division between inner and outer vital conditions, and that we cannot, therefore, make a strictly theoretical distinction between the conception of stimulation and that of the processes of development. I wished to show that, for these reasons, we must not expect from the conception of stimulation, as we understand it, anything beyond its possibilities. But finally I wished also to show that, whilst fully conscious of and with due consideration of all these difficulties, it is possible to work out a definition of stimulation which is of great _practical_ working value. The definition in short is: “_Stimulus is every alteration in the external vital conditions._” This definition gives to the conception of stimulation its most complete, that is to say, its generally applicable and simplest form. The great importance from a methodical standpoint of this definition of stimulation for the research of life is evident. Our whole experimental natural science always employs for investigation of any state or process the same method: the state or process to be observed is studied under systematically altered conditions. By stimulating the living substance it is brought under changed external conditions. A systematic employment of stimulus is, therefore, the experimental means for the research of life. CHAPTER III THE CHARACTERISTICS OF STIMULI