Thus have the opinions handed down by former writers upon Vital Principle been delineated; and now let us retrace our steps, and again, as if at the outset of our inquiry, endeavour to define what it is and what the most general expression for it.
We say, then, that essence is a particular genus of entities, and that of it part is matter, which in itself is not any one particular object, as it is other than form and species from which each object derives its particular denomination; and that, in the third place, there is the derivative from both these. Now matter is potentiality, species reality, and that in a twofold acceptation, as knowledge and as reflexion; but bodies, and above all natural bodies, seem to be essences; for they are, in fact, the origins of other bodies. Among natural bodies some have and some have not life; and by life we mean the faculties of self-nourishment, self-growth and self-decay. Thus, every natural body partaking of life may be regarded as an essence; but then it is an essence in combination, as has been said. And since the body is such a combination, being possessed of life, it cannot be Vital Principle; for as it is itself more truly subject and matter, it cannot be among the subordinates of a subject. It follows, then, that the Vital Principle must be an essence, as being the form of a natural body holding life in potentiality; but essence is a reality,—the reality, that is, of a body such as has been described. Now reality is, in the twofold signification, either of knowledge or of reflexion; and that it may be regarded as knowledge is manifest in that sleep and watching co-exist as original properties, in Vital Principle; and equally manifest that watching is analogous to reflexion upon knowledge, as that sleep represents knowledge possessed but not employed. But knowledge pre-exists in the same individual, and the Vital Principle is, therefore, the original reality of a natural body endowed with life in potentiality; only this is to be understood of a body which may be organised. Thus, the parts even of plants are organs, but then they are organs which are altogether simple, as the leaf is the covering of the pericarp, and the pericarp of the fruit; and the roots are analogous to the mouth, for both take in food. If, then, there be any general expression for every kind of Vital Principle, it may be set down as "the incipient reality of a natural body which is organised"
It is, therefore, to no purpose to inquire whether Vital Principle and the body are one, any more than whether wax and the impress upon it are one, or whether the matter formative of any object and the object formed are one ; for one and being have many significations, but they are correctly designated as reality.
It has thus been explained generally what the Vital Principle is, and shewn that it is an essence, in its abstract signification, which implies the particular mode of being in any particular body, as if any instrument, an axe, for instance, were a natural body, the mode of being in the axe would be, at once, both its essence and its Vital Principle; for, were it once to be withdrawn, then, save in name, it could be an axe no longer. All this, however, relates to an axe, but Vital Principle is the mode and the cause of being, not in any thing like an axe but, in a natural body, having within it a principle of motion and of rest.
But what has been said may be better understood by reference to the parts of a body. Thus, if the eye were an animal, vision would be its Vital Principle, as vision, abstractedly considered, is the essence of the eye; but the eye is the matter of vision, and if vision be wanting, then, save in name, it is an eye no longer, any more than is that an eye which is represented in sculpture or painting. All that has here been assumed of a part may be made applicable to the whole living body; for, as there is an analogy between part and part, so is there between the whole sensibility and the whole sentient body, in the ratio of its sensibility ; but this must be understood of a body which yet retains its Vital Principle, and is, in potentiality, alive. The seed and the fruit are the representatives of such a body in potentiality ; and as cutting is the reality of an axe, vision that of an eye, so watching is the reality of Vital Principle ; which is to the body what vision is to an eye, and its own property to any instrument ; but this is to be understood of a body in potentiality. Thus, as an eye is a pupil and vision, so an animal is a body and Vital Principle.
It is then obvious that neither Vital Principle nor any of its parts, even granting that it may be divisible, can be separate from the body; for of some of its parts it is the reality; and yet there is nothing to preclude the possibility of some others being separate, as there are some which do not contribute to the reality of any body. It is doubtful, however, whether the Vital Principle is the reality of a body in the sense that a mariner is of his vessel.
Thus far, then, have we proceeded in our attempt to define and delineate Vital Principle.