No single condition in a process in the least complex can be considered causal; rather the evolution of a process depends on all conditions being in proper order. All changes in vital conditions act as stimuli. In the present work, however, only changes in the external world are considered as stimuli; changes within the organism are considered as development.
Stimuli are considered as to their amount, quality (negative or positive), intensity, form and rapidity. Under intensity Fechner's law (the intensity of sensation varies with the logarythm of the intensity of the stimulus), and the "all or none law" (any response is the most complete response possible, i. e., the heart beat) are discussed. Stimulation also causes a qualitative change in vital processes metabolically, a practical differentiation from the constant change going on in the metabolism of rest or equilibrium of metabolism. In equilibrium of metabolism assimilation=1, i. e., biotonus. Stimuli