Note:  Do not rely on this information. It is very old.


Life, the subject-matter of biology, has never, perhaps, been satisfactorily defined. One of the earliest, simplest, and best definitions is that of St. Thomas Aquinas, that life is self-movement. Hafeland's "life is the activity of the organic forces," explains little; Bichat's, "the sum-total of the forces which resist death," is purely negative; Kichte's, "the self-sustentation of the organism," is very narrow and one-sided, applying solely to vegetative growth, and Duges's, "the special activity of organised bodies;" and Beclard's, "organisation in action," seem hardly maintainable in the face of the absence of organisation in the lowest animals, such as amoeba. Schelling's, "tendency to individuation," refers rather to structural development than to the vital functions of a mature organism, and might include crystallisation; and Richerand's, "a collection of phenomena which succeed each other during a limited time in an organised body," applies equally to decomposition after death. De Blainville's, "the twofold internal movement of composition and decomposition, at once general and continuous," expresses, as Mr. Herbert Spencer points out, only vegetative life, and is equally applicable to a galvanic battery. Three better and closely-allied definitions are those of Cuvier, Schopenhauer, and Lewes. Cuvier's, "the capacity of the organism for assimilating external elements, and preserving its own identity," refers too exclusively to nutrition. Schopenhauer's, "that condition of a body in which it preserves its essential form, whilst the matter of which it is composed is constantly changing," also too much ignores active life, such as that manifested by muscular and nervous action. G. H. Lewes's, "a series of definite and successive changes, both of structure and composition, which take place within an individual without destroying its identity," seems to deny the obvious fact that many vital processes are not successive but simultaneous. Mr. Spencer first suggests, "the coordination of actions," and then, in order to exclude the action of a glacier, for instance, "the definite combination of heterogeneous changes, both simultaneous and successive." He then points out that we commonly recognise the living by its giving a fit response to an external stimulus, the tree budding or the animal shrinking from the touch, and adds the words "in correspondence with external co-existences and sequences" to his first definition, which he then abbreviates into "the continuous adjustment of internal relations to external relations," a formula little more suggestive of any adequate conception of life than that of Schelling. The facts that life is always manifested in the presence of protoplasm (q.v.) which has been termed "the physical basis of life," a spontaneously decomposing complex admixture of compounds of carbon, hydrogen, oxygen, nitrogen, and sulphur, saturated with water; that living bodies possess the two great powers of assimilation and reproduction, and that they have generally curved outlines, if not also a definite form and some organisation or differentiation of parts, have been dealt with further under Biology (q.v.).