The conception of _excitation_ and _depression_ are purely empirical. They are terms for real things, referring, in fact, simply to alterations in rapidity of life process, which can be as readily observed as the process itself. I wish to lay particular stress on this fact, for the reason that _Cremer_[44] has recently made the extraordinary statement that I have introduced hypothetical processes into the definition of the conception of excitation. I have always considered excitation as merely an increase or change of intensity of the specific actions of a living system, and as such is an established process without a _trace_ of the hypothetical element.[45] If, however, the excitation process is to be regarded as something _absolute_, as a mysterious state _sui generis_, which is entirely independent and totally unlike the metabolism of rest, then, of course, it would appear utterly incomprehensible and would be without purpose. As an _absolute_ process excitation is merely a meaningless word. Excitation and depression are _relative_ conceptions and can only acquire meaning when the process which is excitated or depressed is more closely defined. This is the specific vital process of a given organism, and the two conceptions only have meaning in relation to it. The conception of the vital process, however, is one directly gained from experience. However complex or difficult to analyze the process may be, it still is as little hypothetical as that of the combustion of carbon into carbon dioxide, or the revolving of the earth around the sun. It can be looked upon as something positive and real. Quite another question is the manner in which we are to consider the mechanism of the vital process. In analyzing this mechanism we cannot, at least in the present state of our knowledge, entirely dispense with hypothesis. But these hypotheses are in no way involved in the _definition_ of the process of excitation. If we look upon every excitation or depression produced by a stimulus as an alteration in rapidity in the specific vital process of a given organism, we are thereby expressing the same fact which _Johannes Müller_ has termed “_specific energy_.” We give, however, the doctrine of specific energy a more general application in so far as it comprehends not only the increase but likewise the decrease of activity in response to stimuli. _Johannes Müller’s_ doctrine of specific energy of the living substance at all times has been the subject of most animated discussion. When I refer here to the specific energy of living substance, it is with the knowledge that _Johannes Müller_ did not use this expression of “living substance” in this connection. He was already acquainted, however, as we have seen, with the fact of the existence of the specific energy of all living structures. For appertaining to the muscle he says: “This is universal in all organic reaction.” The reason why the doctrine of _sense energy_