What is Life? Were such a question proposed, we should be tempted to answer, what is _not_ Life that really _is_? Our reason convinces us that the quantities of things, taken abstractedly as quantity, exist only in the relations they bear to the percipient; in plainer words, they exist only in our minds, _ut quorum esse est percipi_. For if the definite quantities have a ground, and therefore a reality, in the external world, and independent of the mind that perceives them, this ground is _ipso facto_ a quality; the very etymon of this world showing that a quality, not taken in its own nature but in relation to another thing, is to be defined _causa sufficiens, entia, de quibus loquimur; esse talia, qualia sunt_. Either the quantities perceived exist only in the perception, or they have likewise a real existence. In the former case, the quality (the word is here used in an active sense) that determines them belongs to Life, _per ipsam hypothesin_; and in the other case, since by the agreement of all parties Life may exist in other forms than those of consciousness, or even of sensibility, the _onus probandi_ falls on those who assert of any quality that it is _not_ Life. For the analogy of all that we know is clearly in favour of the contrary supposition, and if a man would analyse the meaning of his own words, and carefully distinguish his perceptions and sensations from the external cause exciting them, and at the same time from the quantity or superficies under which that cause is acting, he would instantly find himself, if we mistake not, involuntarily identifying the ideas of Quality and Life. Life, it is admitted on all hands, does not necessarily imply consciousness or sensibility; and we, for our parts, cannot see that the irritability which metals manifest to galvanism, can be more remote from that which may be supposed to exist in the tribe of lichens, or in the helvellæ, pezizee, &c., than the latter is from the phenomena of excitability in the human body, whatever name it may be called by, or in whatever way it may modify itself.(8) That the mere act of growth does not constitute the idea of Life, or the absence of that act exclude it, we have a proof in every egg before it is placed under the hen, and in every grain of corn before it is put into the soil. All that could be deduced by fair reasoning would amount to this only, that the life of metals, as the power which effects and determines their comparative cohesion, ductility, &c., was yet lower on the scale than the Life which produces the first attempts of organization, in the almost shapeless tremella, or in such fungi as grow in the dark recesses of the mine.