# On the Vital Principle/Book 2/Chapter 4

It is necessary, in order well to study those faculties, that we should comprehend what each of them individually is, and then, in like manner, carry our inquiry into their consequences and other conditions. But if it behove us to say what each of them is, as what is the cogitative, sentient, or appetitive faculty, it should previously be settled what that is which thinks and that which feels; for energies and acts are, abstractedly considered, pre-existent to their functions. Granting, however, that it is so, and that we ought, before the faculties or functions, to have considered their opposites, it might be fitting here also, and for the same reason, first to define the opposites of the functions define, that is, food before nutrition; the object before perception; and the intelligible before thought.
Thus we must first speak upon nutrition and generation, for the nutritive faculty is innate in other beings besides animals; it is the primal and most universal influence of the Vital Principle, and through it life is manifested in all beings. Its functions are to generate and to employ nourishment; for the most ​natural of the functions in beings which are perfect, that is, which are neither dwarfed nor spontaneously generated, is to produce another such as itself, an animal an animal, and a plant a plant, in order that they may partake, to the extent which has been allotted to them, of the Everlasting and the Divine. All creatures yearn after this, and, for the sake of it, they do all that they do naturally ; but since such beings cannot, in uninterrupted continuity, partake of the Everlasting and the Divine, because no perishable being can abidingly continue as one and the same ; yet each can partake thereof in its own allotted portion, be it larger or smaller, and still continue, if not the same, like the same, and one, if not in number, as species.
The Vital Principle is the cause and the origin of a living body. Now, cause and origin have several significations; for the Vital Principle is equally a cause, according to any one of the three defined modes of causation: as that whence motion proceeds; as that for which motion is produced; and cause, again, as the essence of living bodies. It is evident that it is a cause as an essence, since the essence is in all things the cause of their being what they are; and as life is the mode of being in living beings, so Vital Principle is the cause and the origin of all such. It is the realizing principle, besides, the cause that is of something which exists in potentiality becoming a ​reality. It is manifest, too, that Vital Principle is a cause, in the sense of a final cause; for as the mind acts for some end, so does nature, and that end is her aim; and such an aim has the Vital Principle, by its nature, in living bodies. Thus, all natural bodies, those of animals as well as those of plants, are its instruments, and are what they are for its purposes. The term final cause has a twofold signification, as it implies that for which, as well as that by which, any result is obtained; and Vital Principle is a, final cause, as that whence locomotion is derived, although this is a property which does not belong to all living creatures. Change and growth, moreover, are dependent upon Vital Principle; for sensation seems to be a change of some kind, and whatever is sentient has Vital Principle; and this applies equally to growth and decay, for nothing grows or decays naturally unless it be nourished, and nothing is nourished which does not partake of life. Empedocles has not expressed himself happily upon this point, as, after other observations, he adds that plants take growth downwards, where they strike root, from this being the natural direction of earth, and upwards, from this being the natural direction of fire. Neither has he clearly seized the import of the terms upwards and downwards, as they are not identical for all creatures, or for the universe; for the nead is to animals what the roots are to plants, if we may speak of organs ​after their functions, although in other respects different. But, besides these objections, what is that which is to hold fire and earth, with their opposing tendencies, together? Now, unless there be a restraining force, they must be torn asunder, and if such there be, it ought to be regarded as Vital Principle, and the cause both of nourishment and growth.
The nature of fire seems, to some philosophers, to be the absolute cause of nutrition as well as growth, and that because it alone, among bodies or elements, appears to be nourished and to grow. It might, therefore, be assumed, that it is fire which works out those processes in plants and animals; but although fire is possibly a joint cause, it cannot be the exclusive cause, as this must be assigned rather to the Vital Principle. The increase of fire is infinite, so long as there is any thing combustible, but to all the bodies of nature's constitution there is a limit and a relation both as to bulk and increase; and these are conditions, not of fire but of Vital Principle; not of matter but of design.
Since the same faculty of Vital Principle is at once nutritive and generative, it is necessary first to define nutrition ; for it is by this, compared with other faculties, that Vital Principle is especially distin- guished. Nutrition, then, appears to be a contrary acted upon by a contrary, but this does not imply any kind of contrary by any other contrary; it refers only ​to such contraries as can generate from and give growth to one another. Thus, there are many things derived from one another which are not always quantities, as the healthy, for instance, is derived from the unhealthy; neither do these contraries appear, in any manner, to be nourishment for one another, as water, for instance, is nourishment for fire, but fire is not nourishment for water. It is in homogeneous bodies especially, that the contraries seem to be in the relations of nourishment and nourished. But here there is a difficulty; for while some maintain that like is nourished as it is increased by like, there are others who maintain, as we have said, that it is contrary which is nourished by contrary; that like is unimpressionable by like; that food undergoes change and is digested, and that all-change implies conversion to an opposite or an intermediate state. Nourishment, besides, is affected by the body which is nourished, although the body is not affected by the nourishment, just as the material is affected by the artisan, although he is not affected by the material; for it is the artisan alone who converts the material from a raw state into one of usefulness. There is, however, a distinction to be observed in nourishment, between its last and adventitious or its first state; if both states are nourishment, distinguished only by the one being undigested, and the other digested, then it may be correct to admit of both explanations for nutrition; for in so ​far as food is undigested, it is contrary nourished by contrary, and in so far as it is digested, it is like nourished by like. Thus, it is manifest that both these opinions are in one sense right, and in another wrong. But as nothing can be nourished which does not partake of life, so a living body may be regarded as a body which is nourished from having life; and thus nutrition is not in a casual, but a positive relation to a living body. There is an obvious distinction between nourishment and growth: in so far as a living body is quantity, it is capable of growth, and in so far as a something is matter and essence, it is nourishment; for it preserves the essence of the body, which exists so long as it can be nourished. Nourishment, however, does not generate that which is nourished, as it is the same as it; for it is already itself the essence, and nothing can generate, although it may preserve itself. Thus, it is the same faculty of Vital Principle which is able to preserve that, such as it may be, which contains it, and it is nourishment which renders it fit for its office; and, therefore, when deprived of nourishment, it can exist no longer.
Now, there are here three things or conditions—something to be nourished, something by which nourished, and something which nourishes. That which nourishes is the primal or nutritive faculty; that which is nourished is the body; and that by which nourished is food. And as things are correctly ​designated after the object to which they tend, and as the object here is to generate another like itself, so the primal faculty may be set down as being generative of another like itself. That “by which nourished” has a twofold signification, as has that by which a vessel is steered, and which implies hand and rudder, of which the one only moves, while the latter both moves and is moved. It is necessary to nutrition that food should admit of being digested, and as it is heat which works out digestion, so all living creatures have heat.
It has thus then be shewn, although but superficially, what nutrition is; but the subject shall be further elucidated in other treatises upon the subject.

## Notes

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​Note 1, p. 77. But since such beings cannot, &c.] The purport of this passage is almost too obvious for comment, embodying the great fact of the perpetuation of the species, and compensation, by reproduction, for the death of the individuals; and number refers, of course, to individuals, species to the aggregate.
Note 2, p. 78. The term final cause, &c.] This is a kind of parenthetical clause, intended merely to guard against the supposition that the fact of some animals having a fixed habitat, not being locomotive that is, was unknown, or had escaped notice.
Note 3, p. 78. And this applies equally to growth and decay.] Aristotle perceived, although it may be indistinctly, that the source of nutrition is through the blood—he perceived, that is, that "the blood is replenished by vessels, which arise on and are spread over the mesentery, and which empty themselves into the cava and the ​aorta;" the anatomy is, no doubt, imperfect, but it still is an outline of the knowledge of the lacteals. It seems to shew that veins gather fluid from the intestines, and convey it to the large blood vessels; but there was no analogous knowledge, in that age, whereby the process of decay that is absorption could be accounted for. The term decay, therefore, was the mere expression for a general fact.
The objection to the terms "upwards and downwards," used by Empedocles to delineate the growth of plants, suggests the advantage that would accrue to science, if its terms were made sufficiently precise to fix, beyond doubt, the several relations and positions of the same body, or all bodies. And in the analogy between the heads of animals and roots of trees, we cannot but perceive the outline of a doctrine which has been developed, by modern science, into homologous physiology.
Note 4, p. 79. The nature of fire seems to some philosophers, &c.] This is an argument, drawn from the agency of fire, to disprove the then prevailing opinion, that, as it alone of the elements appears to be nourished and to grow, it may be the source of life and the origin of living actions; as they are shewn, by the contrast between living and igneous properties, to be essentially distinct from one another. The opinion may have originated from the fact that heat accompanies digestion, and as fire was by some held to be the first element, it was readily supposed to be the agent in that process. As ​an illustration of this opinion, it was maintained, even by Aristotle, that "food, taken into its appointed receptacles, is vaporised and transmitted to the veins, in which, undergoing change, it is converted into blood, and carried onward to the heart."
Note 5, p. 81. There are here three things, &c.] The meaning of this passage, apart from its scientific wording, is sufficiently obvious—that which nourishes is food when digested; for food both acts and is acted upon by the body, and, when so acted upon, it is assimilated to and incorporated with its substance, through the blood. But food, being dead, is contrary to the living matter, which has, however, power to convert it into like, to assimilate it, that is, to the living system. Thus, food, in its first state, is contrary to or unlike, and in its last, or concocted state, it is like the body; and, therefore, the same element is in one sense contrary, and in another sense like, acting upon a contrary or like. So too the rudder, which directs the vessel, represents the stomach, which converts the food into nourishment for the body; and the sensibility, which gives power to the stomach, represents the hand which, through the rudder, directs the motion of the vessel; and the vessel is analogous to the body which is nourished.
* ↑ De Part. Animalm, IV. 3. 4.
* ↑ De Somno, 3. 3.