If it were asked, to what purpose or with what view we should generalize the idea of Life thus broadly, I should not hesitate to reply that, were there no other use conceivable, there would be _some_ advantage in merely destroying an arbitrary assumption in natural philosophy, and in reminding the physiologists that they could not hear the life of metals asserted with a more contemptuous surprise than they themselves incur from the vulgar, when they speak of the Life in mould or mucor. But this is not the case. This wider view not only precludes a groundless assumption, it likewise fills up the arbitrary chasm between physics and physiology, and justifies us in using the former as means of insight into the latter, which would be contrary to all sound rules of ratiocination if the powers working in the objects of the two sciences were absolutely and essentially diverse. For as to abstract the idea of _kind_ from that of _degrees_, which are alone designated in the language of common use, is the first and indispensable step in philosophy, so are we the better enabled to form a notion of the _kind_, the lower the _degree_, and the simpler the form is in which it appears to us. We study the complex in the simple; and only from the intuition of the lower can we safely proceed to the intellection of the higher degrees. The only danger lies in the leaping from low to high, with the neglect of the intervening gradations. But the same error would introduce discord into the gamut, _et ab abusu contra usum non valet consequentia_. That these degrees will themselves bring forth secondary kinds sufficiently distinct for all the purposes of science, and even for common sense, will be seen in the course of this inquisition: for this is one proof of the essential vitality of nature, that she does not ascend as links in a suspended chain, but as the steps in a ladder; or rather she at one and the same time _ascends_ as by a climax, and expands as the concentric circles on the lake from the point to which the stone in its fall had given the first impulse. At all events, a contemptuous rejection of this mode of reasoning would come with an ill grace from a medical philosopher, who cannot combine any three phenomena of health or of disease without the assumption of powers, which he is compelled to deduce without being able to demonstrate; nay, even of material substances as the _vehicles_ of these powers, which he can never expect to exhibit before the senses.