By a large preliminary simplification, Life may be defined as _the mode of existence of an organism in relation to its medium_. To render this of any value, however, a clear conception of the organism is first indispensable; and this must be preceded by an examination of the various attempts to define life in anticipation of such a clear conception. 26. Every phenomenon, or group of phenomena, may be viewed under two aspects--the _statical_, which considers the conditions of existence; and the _dynamical_, which considers these conditions in their resultant,--in their _action_. The statical definition of Life will express the connexus of the properties of organized substance, all those conditions, of matter, form, and texture, and of relation to external forces, on which the organism depends. These various conditions, condensed into a single symbol, constitute Vitality or Vital Force, and are hence taken as the Cause of vital phenomena. The dynamical definition will express the connexus of Functions and Faculties of the organism, which are the statical properties of organized substance in action, under definite relations. It is obvious that the term Life must vary with the varying significates it condenses,--every variation in the complexity of the organism will bring a corresponding fulness in the signification of the term. The life of a plant is less significant than the life of an animal; and the life of a mollusc less than that of a fish. But not only is the term one of varying significance, it is always an abstract term which drops out of sight particular concrete differences, registering only the universal resemblances. * * * * * 27. It would be a profitless labor to search out, and a wearisome infliction to set down, the various definitions which have been proposed and accepted; but certain characteristic examples may be selected. All that I am acquainted with belong to two classes: 1°, the _meta_-physiological hypothesis of an _extra_-organic agent, animating lifeless matter by unknown powers; 2°, the physiological hypothesis which seeks the cause of the phenomena (i. e. the conditions) _within_ the organism itself,--a group of conditions akin to those manifested elsewhere, but differently combined. The first hypotheses are known under the names of Animism and Vitalism,--more commonly the latter. The second are known as Organicism and Materialism,--but the latter term only applies to some of the definitions.