All the faculties of Vital Principle which have been enumerated belong, as we have said, to some creatures, some only of them belong to others, and there are creatures again which have but one; and we spoke of those faculties as the nutritive, appetitive, sentient, locomotive and cogitative. Of these, the nutritive alone belongs to plants; but to other beings both it and the sentient have been imparted; and if the sentient, then the appetitive, for appetite is desire, passion and volition; and all animals, without exception, have the sense of Touch. But the creature to which sensibility has been imparted cannot but be sensible of pleasure and pain, of what is grateful and what painful; and if sensible of these, it must have desire, as desire is the appetite for what is grateful. All such creatures, moreover, have the sense for food, as they have Touch, which is that sense; for all animals are nourished by what is dry and moist, warm and cold, and Touch is the sense for judging of these qualities. But it is only by chance that the Touch can judge of other qualities, as neither sound, colour nor odour contribute in aught to nourishment; and savour is among tangible qualities. Hunger and thirst are desires: the former for what is dry and warm, the latter for what is liquid and cold ; and savour is the condiment, as it were, for both. As, however, we shall be more explicit upon those points hereafter, it may, for the present, suffice to say, that all such creatures as have the sense of Touch have appetite ; it is uncertain whether or not they have imagination, but this also shall be considered hereafter. There are creatures to which, besides those faculties, locomotion has been imparted; and others again, as man, to which have been allotted both reflexion and mind, together with any other and yet nobler faculty, if such there be, than mind.
It is clear, then, that there can be but one definition for Vital Principle, as there is but one for a geometrical figure; for as in geometry there is no figure but the triangle and its sequences, so neither are there any kinds of Vital Principle save those which have been enumerated. Could there, however, be any such common expression for figures, as without being peculiar to any one, should yet be applicable to all, so might there be for the Vital Principles alluded to. It would be idle, however, to seek for any such expression, in the case either of Vital Principles or geometrical figures, as should neither be applicable to any one of them individually, nor, putting aside individuals, be applicable to them as an individual species. But still there is an analogy between the faculties of Vital Principle and geometrical figures; for as in vital properties, so in geometrical figures, the antecedent is ever present potentially in the sequences, and as the triangle is in the square, so the nutritive is in the sentient faculty. Thus, the inquiry must be conducted with reference to individuals, in order to learn what is the Vital Principle of each, as of a plant, a man, or a brute; and wherefore beings are thus ranged in a series.
Without the nutritive function there can be no sensibility, but in plants the nutritive exists without the sentient; so again without the Touch there can be no other sense, while Touch can exist alone, for many animals have neither sight nor hearing, and are altogether without smell. Among sentient creatures some have and some have not locomotion, and, finally, to a few calculation and judgment have been imparted; and to such among mortal beings as are so endowed all other faculties have been imparted likewise. But to such as possess some one only of the faculties, calculation has not been allotted, as some of them have not even imagination, while others live by it alone; it would be foreign to our present inquiry to enter upon the speculative intellect.
It is, then, clear that the definition which comes closest to each one of those faculties is also the fittest for the elucidation of Vital Principle.