This application has been found legitimate, and has been justified by experiment whenever it has been a question of the laws or of the really fundamental or elementary conditions of phenomena. It has, on the other hand, however, been unfortunate when it has stopped short of secondary characteristics. When we now concede the subjection of living beings to these general laws of energetics, we are following a traditional method. There is no doubt that this application is legitimate, and that experiment will justify it _a posteriori_. I will therefore grant, as a provisional _postulate_, the consequences of which will have to be ultimately justified, that the living and inanimate world alike show us nothing but _transformations of matter_ and _transformations of energy_. The word phenomenon will have no other signification, whatever be the circumstances under which the phenomenon occurs. The varied manifestations which translate the activity of living beings thus correspond to transformations of energy, to conversions of one form into another, in conformity with the rules of equivalence laid down by the physicists. This conception may be formulated in the following manner:—_The phenomena of life have the same claim to be energetic metamorphoses as the other phenomena of nature_. This postulate is the foundation of biological energetics. It may be useful to give some explanation relative to the signification, the origin, and the scope of this statement. Biological energetics is nothing but general physiology reduced to the principles that are common to all the physical sciences. Robert Mayer and Helmholtz gave the best description of this science, and laid down its limits by defining it as “the study of the phenomena of life regarded from the point of view of energy.” § 1. ENERGY AT PLAY IN LIVING BEINGS. COMMON OR PHYSICAL ENERGIES. VITAL ENERGIES. Our first object will be to define and to enumerate the energies at play in living beings; to determine their more or less easy transformations from one to another, to bring to light the general laws which govern those transformations, and finally to apply them to the detailed study of phenomena. This programme may be divided into four parts. In the physical world the specific forms of energy are not numerous. When we have mentioned mechanical, chemical, radiant (thermal and photic) energies, electrical energy, with which is blended magnetic energy, we have exhausted the catalogue of natural agents. But is this list for ever closed? Are vital energies comprised in this list? These are the first questions which we must ask ourselves.