The “life” that is found in human bodies and throughout the organic world is impersonal. It is material, or, rather, physical, in the sense that it has not any existence apart from the organic matter of which it constitutes an affection or attribute. The case is analogous to that of gravitation. According to the accepted Newtonian philosophy every particle of matter in the universe attracts, and is attracted by, every other particle: it gravitates: it is ponderable. But there is no such thing as gravitation _per se_, though there may be an entity that causes gravitation. It is convenient, for the purposes of language and the orderly expression of thought, to speak of it separately, just as colour, temperature, illumination, form, structure and other affections are referred to; the understanding, however, being always that they are not in themselves entities and are not characterised by anything in the nature of self-existence. Science does not at present hold that life is an attribute of all matter. Minerals and other forms of what is called inorganic matter are considered to be devoid of life, and the same destitution is asserted with respect to “dead,” “inanimate” organic matter. Life is met with only in “living,” “animate” organic matter; just as crystallisation is found only in “crystalline” and not in “amorphous” matter; and--to pursue the simile--it may be pointed out that the same matter which is crystalline under some conditions becomes amorphous under others, as in the case of carbon, which is sometimes diamond and sometimes charcoal. In like manner living matter may change into dead matter--a change which is called “death”; and dead matter may change into living matter, as, for example, when food is assimilated by animals and vegetables. The true nature of the “life” met with in living organic matter is not yet understood. Modern science has shown, indisputably, that the doctrine of the conservation of energy applies without modification to living beings just as much as to inanimate substances. The idea of there being any specific “vital force,” “vital material” or “vital energy” has long ago been abandoned. All the particular phenomena observed by morphologists, physiologists, embryologists, palæontologists and ætiologists--_i.e._ by the whole world of biologists--can be satisfactorily explained in terms of chemistry, physical force, energy and dynamics. But biology cannot as yet give an equally clear account of the co-ordinated vitality of anything that lives. It cannot even state the how and why of the simplest unicellular organism. “We are forced,” says a leading authority, “to the conclusion that a living organism is a particular synthesis of matter and energy, the secret of whose organisation remains hidden.”