Herbert Spencer says, "Life is the continuous adjustment of internal relations to external conditions." The molecules of the substances forming the living material are large, complex and unstable, and as such they constantly tend to pass from the complex to the simple, from unstable to stable equilibrium.

The elementary substances which form living material are known, but it has hitherto not been found possible artificially so to combine these substances that the resulting mass will exhibit those activities which we call the phenomena of life.

The distinction between living and nonliving matter is manifest only when the sum of the activities of the living matter is considered; any single phenomenon of the living may appear also in the non-living material.

Probably the most distinguishing criterion of living matter is found in its individuality, which undoubtedly depends upon differences in structure, whether physical or chemical, between the different units.
Certain conditions are essential for the continued existence of living matter.

It must be surrounded by a fluid or semi-fluid medium in order that there may be easy interchange with the environment.