Nevertheless, the conception of the metabolism of rest as an abstraction can be of value provided always that it is strictly and definitely limited. It must, for instance, not be applied to short periods of time. The continued local and temporary responses to stimulation constitute a mean value which, although composed of numberless small sub-threshold responses, we can still call a metabolism of rest. Weak stimuli have, however, as already seen, the property, provided their influence is constant, of effecting an adaptation to the stimulus on the part of the living organism, so that the stimulus becomes a vital condition for this state of the organism. Hence the continued existence of a vital process resulting from the constant action of stimulation is made possible. That which we are in the habit of calling metabolism of rest, would, therefore, be metabolism of stimulation, but one that is characterized by a constantly existing metabolic equilibrium. This “_equilibrium of metabolism_” distinguishes the metabolism of rest from that form which is developed in response to temporary stimulation, in that every temporary stimulation has the effect that it disturbs the existing metabolic equilibrium for a longer or shorter time. This disturbance of the equilibrium of metabolism can in contrast to the metabolism of rest be termed “_metabolism of stimulation_.” In this, but only in this sense, can these two conceptions be placed in opposition and used to characterize the processes in the living organism. The conception of the metabolism of stimulation must always stand in relation to that of an equilibrium of metabolism characterized by a constantly existing metabolism of rest, just as the conception of stimulus can likewise only be defined relatively to that of vital conditions. Nevertheless, the conception of the equilibrium of metabolism requires a somewhat more accurate definition before we can feel justified in using this term. Definitions are always trite, nevertheless they are the basis of all our thinking and a definite understanding is impossible unless we first clearly fix their contents. The history of theology and philosophy even to the most recent times furnishes a long line of instances in which the most eminent minds, for the want of fixed definitions of the conceptions which they made use of, failed to find a mutual basis for their ideas. Without a sharp definition every conception is a mere word, which each individual, according to his personal experiences and views, endows with a different meaning. To such conceptions we may apply Mephisto’s ironical comment to his pupil: “Mit Worten lässt sich trefflich streiten, Mit Worten ein System bereiten.”