Élan vital
Élan vital is the physical quantity responsible for motion in the cosmos of Usm. Though it is sometimes referred to as a substance or as a form of matter, élan vital does not have most of the properties of other matter, and cannot be converted to or from inert matter.
Élan vital exists in a number of different forms, and can be changed between them. The form of élan vital directly responsible for motion is élan motif; the momentum of an object is directly proportional to the amount of élan motif contained within it. Élan motif has no mass of its own. There is another form of élan vital, however, élan corporel, which does. The bodies of most living creatures are made primarily of élan corporel (bones and shells and certain other parts excepted), and in order to move the creature converts some of that élan corporel into élan vital. (The mass of the creature does not change significantly because a very small amount of élan corporel yields sufficient élan motif for a very large momentum.)
Another important form of élan vital is the élan âmal. It is the élan âmal that makes up what is called a creature's "soul"; the difference between a corpse and a living being is that the latter possesses élan âmal, and the former does not. Other forms of élan vital also exist--many extraplanar creatures, for instance, are made up largely of yet another form, élan lutin, and demons (and certain similar entities) consist mostly of a form called élan génial.
Whether or not élan vital is conserved remains an open question. It's true that under most circumstances, élan vital, while it may be altered in form, is neither created or destroyed, but there are a few apparent exceptions. It has yet to be decisively demonstrated, for instance, what happens to a living creature's élan êmal upon death (setting aside those rare occasions when it remains in place as the animating force of an undead being). Some scientists believe that it splinters into the élan âmal of numerous micröorganisms; others suggest that it dissolves into an intangible form of élan vital not yet understood. Neither of these hypotheses, however, nor any of the many others that have been proposed, has yet found any experimental evidence to back it up. Another possible instance in which élan vital may not be conserved involves gravity, which seems to create élan motif ex nihilo. Again, suggestions have been made as to where the apparently new élan vital may actually have come from, but none has been proven. It has even been suggested that these two phenomena are related, and that the élan âmal from dying organisms somehow becomes the gravitational élan motif, but no really satisfactory mechanism for this has been proposed. For the moment, the possibility remains, as unpalatable as many scholars find it, that élan vital really is being created and destroyed, and is not a conserved quantity.
Élan vital is the source not only of motion in Usm, but also of magic. All organisms can manipulate élan vital to some extent--they do so to move and to grow, converting élan corporel to élan motif for the former purpose, and élan vital of various kinds taken by feeding into élan corporel for the latter. But usually this conversion of élan vital is done automatically and unconsciously. It is possible, however, to learn to control the élan vital more consciously, achieving effects that may be impossible through other means. This process is called magic, and can be very dangerous to the inexperienced practitioner due to the risks of converting too much of one's élan corporel or even élan âmal before it can be replenished.