The quest of relations of this last kind is the most important function of special experimental research, because we are enabled by it to complete in thought facts that are only partly given. It is self-evident that only experience can ascertain that between the [alpha beta gamma ...] relations exist and of what kind they are. Further, only experience can tell that the relations that exist between the [alpha beta gamma ...] are such that changes of them can be reversed. If this were not the fact all occasion for the enunciation of the principle of energy, as is easily seen, would be wanting. In experience, therefore, is buried the ultimate well-spring of all knowledge of nature, and consequently, in this sense, also the ultimate source of the principle of energy. But this does not exclude the fact that the principle of energy has also a logical root, as will now be shown. Let us assume on the basis of experience that one group of sensuous elements [alpha beta gamma ...] determines _uniquely_ another group [lambda mu nu ...]. Experience further teaches that changes of [alpha beta gamma ...] can be _reversed_. It is then a logical consequence of this observation, that every time that [alpha beta gamma ...] assume the same values this is also the case with [lambda mu nu ...]. Or, that purely _periodical_ changes of [alpha beta gamma ...] can produce no _permanent_ changes of [lambda mu nu ...]. If the group [lambda mu nu ...] is a mechanical group, then a perpetual motion is excluded. It will be said that this is a vicious circle, which we will grant. But psychologically, the situation is essentially different, whether I think simply of the unique determination and reversibility of events, or whether I exclude a perpetual motion. The attention takes in the two cases different directions and diffuses light over different sides of the question, which logically of course are necessarily connected. Surely that firm, logical setting of the thoughts noticeable in the great inquirers, Stevinus, Galileo, and the rest, which, consciously or instinctively, was supported by a fine feeling for the slightest contradictions, has no other purpose than to limit the bounds of thought and so exempt it from the possibility of error. In this, therefore, the logical root of the principle of excluded perpetual motion is given, namely, in that universal conviction which existed even before the development of mechanics and co-operated in that development.