1. Definitions of Life.—We have seen in the preceding chapter that all attempts to obtain a distinct conception of the nature of Life in general have ended in failure, and produced nothing beyond a negative result. And the conjecture may now naturally occur, that the cause of this failure resides in an erroneous mode of propounding to ourselves the problem. Instead of contemplating Life as a single Idea, it may perhaps be proper to separate it into several component notions: instead of seeking for one cause of all vital operations, it may be well to look at the separate vital functions, and to seek their causes. When the view of this possibility opens upon us, how shall we endeavour to verify it, and to take advantage of it?
Let us, as one obvious course, take some of the attempts which have been made to define Life, and let us see whether they appear to offer to us any analysis of the idea into component parts. Such definitions, when they proceed from men of philosophical minds, are the ultimate result of a long course of thought and observation; and by no means deserve to be slighted as arbitrary selections of conditions, or empty forms of words.
2. Life has been defined by Stahl[60], ‘The condition by which a body resists a natural tendency to chemical changes, such as putrefaction.’ In like manner, M. von Humboldt[61] defines living bodies to be ‘those which, notwithstanding the constant operation [196] of causes tending to change their form, are hindered by a certain inward power from undergoing such change.’ The first of these definitions amounts only to the assertion, that vital processes are not chemical; a negative result, which we may accept as true, but which is, as we have seen, a barren truth. The second appears to be, in its import, identical with the first. An inward principle can only be understood as distinguished from known external powers, such as mechanical and chemical agencies. Or if, by an internal principle, we mean such a principle as that of which we are conscious within ourselves, we ascribe a soul to all living things: an hypothesis which we have seen is not more effective than the former in promoting the progress of biological science. Nearly the same criticism applies to such definitions as that of Kant: that ‘Life is an internal faculty producing change, motion, and action.’
[60] Treviranus, Biologie, p. 19. Stahlii, Theor. Med. p. 254.
[61] Aphorismen aus d. Chem. Physiol. der Pflanzen, s. 1.
Other definitions refer us, not to some property residing in the whole of an organized mass, but to the connexion and relation of its parts. Thus M. von Humboldt[62] has given another definition of a living body: that ‘it is a whole whose parts, arbitrarily separated, no longer resist chemical changes.’ But this additional assertion concerning the parts, adds nothing of any value to the definition of the whole. And in some of the lower kinds of plants and animals it is hardly true as a fact.
[62] Versuche über die gereitzte Muskel und Nervenfüser, b. ii. p. 433.
3. Another definition[63] places the character of Life in ‘motions serviceable to the body moved.’ To this it has been objected[64], that, on this definition, the earth and the planets are living bodies. Perhaps it would be more philosophical to object to the introduction of so loose a notion as that of a property being serviceable to a body. We might also add, that if we speak of all vital functions as motions, we make an assumption quite unauthorized, and probably false.
[63] Erhard, Röschlaub’s Magazin der Heilkunde, b. i. st. 1. p. 69.
[64] Treviranus, Biologie, p. 41.