explain the nature of Life, which have fallen within my knowledge, presuppose the arbitrary division of all that surrounds us into things with life, and things without life—a division grounded on a mere assumption. At the best, it can be regarded only as a hasty deduction from the first superficial notices of the objects that surround us, sufficient, perhaps, for the purpose of ordinary discrimination, but far too indeterminate and diffluent to be taken unexamined by the philosophic inquirer. The positions of science must be tried in the jeweller's scales, not like the mixed commodities of the market, on the weigh-bridge of common opinion and vulgar usage. Such, however, has been the procedure in the present instance, and the result has been answerable to the coarseness of the process. By a comprisal of the
petitio principii with the
argumentum in circulo,—in plain English, by an easy logic, which begins with begging the question, and then moving in a circle, comes round to the point where it began,—each of the two divisions has been made to define the other by a mere reassertion of their assumed contrariety. The physiologist has luminously explained Y plus X by informing us that it is a somewhat that is the antithesis of Y minus X; and if we ask, what then is Y-X? the answer is, the antithesis of Y+X,—a reciprocation of great service, that may remind us of the twin sisters in the fable of the Lamiæ, with but one eye between them both, which each borrowed from the other as either happened to want it; but with this additional disadvantage, that in the present case it is after all but an eye of glass. The definitions themselves will best illustrate our meaning. I will begin with that given by Bichat. “Life is the sum of all the functions by which death is resisted,” in which I have in vain endeavoured to discover any other meaning than that life consists in being able to live. This author, with a whimsical gravity, prefaces his definition with the remark, that the nature of life has hitherto been sought for in
abstract considerations; as if it were possible that four more inveterate abstractions could be brought together in one sentence than are here assembled in the words, life, death, function, and resistance. Similar instances might be cited from Richerand and others. The word Life is translated into other more learned words; and this
paraphrase of the
term is substituted for the
definition of the
thing, and therefore (as is always the case in every
real definition as contra-distinguished from a
verbal definition,) for at least a partial
solution of the
fact. Such as these form the
first class.—The second class takes some one particular function of Life common to all living objects,—nutrition, for instance; or, to adopt the phrase most in vogue at present, assimilation, for the purposes of reproduction and growth. Now this, it is evident, can be an appropriate definition only of the very lowest species, as of a Fungus or a Mollusca; and just as comprehensive an idea of the mystery of Life, as a Mollusca might give, can this definition afford. But this is not the only objection. For,
first, it is not pretended that we begin with seeking for an organ evidently appropriated to nutrition, and then infer that the substance in which such an organ is found
lives. On the contrary, in a number of cases among the obscurer animals and vegetables we infer the organ from the pre-established fact of its life.
Secondly, it identifies the process itself with a certain range of its forms, those, namely, by which it is manifested in animals and vegetables. For this, too, no less than the former, presupposes the arbitrary division of all things into not living and lifeless, on which, as I before observed, all these definitions are grounded. But it is sorry logic to take the proof of an affirmative in one thing as the proof of the negative in another. All animals that have lungs breathe, but it would be a childish oversight to deduce the converse, viz. all animals that breathe have lungs. The theory in which the French chemists organized the discoveries of Black, Cavendish, Priestly, Scheele, and other English and German philosophers, is still, indeed, the reigning theory, but rather, it should seem, from the absence of a rival sufficiently popular to fill the throne in its stead, than from the continuance of an implicit belief in its own stability. We no longer at least cherish that intensity of faith which, before Davy commenced his brilliant career, had not only identified it with chemistry itself, but had substituted its nomenclature, even in common conversation, for the far more philosophic language which the human race had abstracted from the laboratory of Nature. I may venture to prophecy that no future Beddoes will make it the corival of the mathematical sciences in demonstrative evidence. I think it a matter of doubt whether, during the period of its supposed infallibility, physiology derived more benefit from the extension, or injury from the misdirection, of its views. Enough of the latter is fresh in recollection to make it but an equivocal compliment to a physiological position, that it must stand or fall with the corpuscular philosophy, as modified by the French theory of chemistry. Yet should it happen (and the event is not impossible, nor the supposition altogether absurd,) that more and more decisive facts should present themselves in confirmation of the metamorphosis of elements, the position that life consists in assimilation would either cease to be distinctive, or fall back into the former class as an identical proposition, namely, that Life, meaning by the word that sort of growth which takes place by means of a peculiar organization, consists in that sort of growth which is peculiar to organized life.
Thirdly, the definition involves a still more egregious flaw in the reasoning, namely, that of
cum hoc, ergo propter hoc (or the assumption of causation from mere coexistence); and this, too, in its very worst form. For it is not
cum hoc solo, ergo propter hoc, which would in many cases supply a presumptive proof by induction, but
cum hoc, et plurimis aliis, ergo propter hoc! Shell, of some kind or other, is common to the whole order of testacea, but it would be absurd to define the
vis vitæ of testaceous animals as existing in the shell, though we know it to be the constant accompaniment, and have every reason to believe the constant effect, of the specific life that acts in those animals. Were we (
argumenti causá) to imagine shell coextensive with the organized creation, this would produce no abatement in the falsity of the reasoning. Nor does the flaw stop here; for a physiological, that is a real, definition, as distinguished from the verbal definitions of lexicography, must consist neither in any single property or function of the thing to be defined, nor yet in all collectively, which latter, indeed, would be a history, not a definition. It must consist, therefore, in the
law of the thing, or in such an
idea of it, as, being admitted, all the properties and functions are admitted by implication. It must likewise be so far
causal, that a full insight having been obtained of the law, we derive from it a progressive insight into the necessity and
generation of the phenomena of which it is the law. Suppose a disease in question, which appeared always accompanied with certain symptoms in certain stages, and with some one or more symptoms in all stages—say deranged digestion, capricious alternation of vivacity and languor, headache, dilated pupil, diminished sensibility to light, &c.—Neither the man who selected the one constant symptom, nor he who enumerated all the symptoms, would give the scientific definition
talem scilicet, quali scientia fit vel datur, but the man who at once named and defined the disease hydrocephalus, producing pressure on the brain. For it is the essence of a scientific definition to be causative, not by introduction of imaginary somewhats, natural or supernatural under the name of causes, but by announcing the law of action in the particular case, in subordination to the common law of which all the phenomena are modifications or results.
Now in the definition on which, as the representative of a whole class, we are now animadverting, a single effect is given as constituting the cause. For nutrition by digestion is certainly necessary to life, only under certain circumstances, but that life is previously necessary to digestion is absolutely certain under all circumstances. Besides, what other phenomenon of Life would the conception of assimilation, per se, or as it exists in the lowest order of animals, involve or explain? How, for instance, does it include sensation, locomotion, or habit? or if the two former should be taken as distinct from life, toto genere, and supervenient to it, we then ask what conception is given of vital assimilation as contradistinguished from that of the nucleus of a crystal?
Lastly, this definition confounds the Law of Life, or the primary and universal form of vital agency, with the conception, Animals. For the kind, it substitutes the representative of its degrees and modifications. But the first and most important office of science, physical or physiological, is to contemplate the power in kind, abstracted from the degree. The ideas of caloric, whether as substance or property, and the conceptions of latent heat, the heat in ice, &c., that excite the wonder or the laughter of the vulgar, though susceptible of the most important practical applications, are the result of this abstraction; while
