We have emphasized that living matter is continually changing, and this fundamental fact is reflected in nearly all attempts to define life. Aristotle described life as "the assemblage of operations of nutrition, growth, and destruction"; deBlainville, as a "twofold internal movement of composition and decomposition"; and Spencer, as "the continuous adjustment of internal relations to external relations."
This interaction consists of chemical and physical processes in which combustion, or oxidation, plays the chief role. Over a century ago it was shown that animal heat results from a slow burning of the materials of the body, involving the consumption of oxygen and the liberation of carbon dioxide ; and further, that for a given consumption of oxygen and liberation of carbon dioxide, about the same amount of heat is produced by an animal as by a burning candle. In other words, the oxidation of the complex compounds which enter the body as food is definitely proportional to the amount of energy which the body gives out, just in the same way as the amount of work performed by a steam engine and the amount of heat it liberates bear a strict proportion to its consumption of fuel.
This is an important discovery, because it goes far toward establishing the fact that at least certain characteristic vital phenomena are in accord with the laws which hold in the non-living world. But the processes of metabolism are not so simple as perhaps might be imagined from the results just mentioned. Heat represents but one of the many energy transformations within the organism, and biologists are at work trying to interpret one after another in terms that are equally applicable in the realms of the living and the lifeless. ' The symbol of the organism is the burning bush of old."
One naturally asks whether living matter possesses some special form of energy â€” 'vital force' â€” which is quite different from chemical and physical energy. This is the philosophically important question of vitalism. From the standpoint of biology we may say that no instrument ever devised has detected such energy, and until some unique vital energy can be made evident to one of the human senses, it does not fall within the scope of science â€” science can neither deny nor affirm its existence. Perhaps for the present it is sufficient to realize that unique phenomena may emerge from new relationships â€” relationships change the properties of things. The properties of molecules are those which the atoms have when they are in the molecule, and the phenomena of life depend on â€” emerge from â€” the physico-chemical constituents of protoplasm when, and only when, they are in protoplasm. A living cell exhibits "many unpredictable properties beyond those of the mere sum of its individual constituent molecules and compounds, or the additive resultants to be derived from any arrangement of them."
However, it is important to note that many of the grosser phenomena of life are being gradually restated in terms of the physical sciences. So it appears clear that the organism is a system for transforming energy into work performed â€” transforming the potential energy stored in chemical complexes of its own substance into the various vital processes of life. And it is in this transfer of energy from one kind to another that we find exhibited the activities which are most distinctive of living things. In these processes of metabolism many complex substances rich in potential energy, which have entered as food and have been, in whole or part, added to the protoplasmic system, are reduced to simpler and simpler conditions and finally, with their energy content nearly or entirely exhausted, are eliminated as excretions. Obviously, if life is to persist, this continual waste must be counterbalanced by a proportionate intake of food in order to renew the supply of energy and to provide the materials which, after preliminary changes, are made into an integral part of the living organism.