As for the thinkers, the Teacher went on, outside our own system of thought, they have, with all their diverse ways of looking at things, one in one point, another in another, approached and touched the doctrine of the Resurrection: while they none of them exactly coincide with us, they have in no case wholly abandoned such an expectation. Some indeed make human nature vile in their comprehensiveness, maintaining that a soul becomes alternately that of a man and of something irrational; that it transmigrates into various bodies, changing at pleasure from the man into fowl, fish, or beast, and then returning to human kind.
While some extend this absurdity even to trees and shrubs, so that they consider their wooden life as corresponding and akin to humanity, others of them hold only thus much— that the soul exchanges one man for another man, so that the life of humanity is continued always by means of the same souls, which, being exactly the same in number, are being born perpetually first in one generation, then in another. As for ourselves, we take our stand upon the tenets of the Church, and assert that it will be well to accept only so much of these speculations as is sufficient to show that those who indulge in them are to a certain extent in accord with the doctrine of the Resurrection.
Their statement, for instance, that the soul after its release from this body insinuates itself into certain other bodies is not absolutely out of harmony with the revival which we hope for. For our view, which maintains that the body, both now, and again in the future, is composed of the atoms of the universe, is held equally by these heathens. In fact, you cannot imagine any constitution of the body independent of a concourse of these atoms. But the divergence lies in this: we assert that the same body again as before, composed of the same atoms, is compacted around the soul; they suppose that the soul alights on other bodies, not only rational, but irrational and even insensate; and while all are agreed that these bodies which the soul resumes derive their substance from the atoms of the universe, they part company from us in thinking that they are not made out of identically the same atoms as those which in this mortal life grew around the soul.
Let then, this external testimony stand for the fact that it is not contrary to probability that the soul should again inhabit a body; after that however, it is incumbent upon us to make a survey of the inconsistencies of their position, and it will be easy thus, by means of the consequences that arise as we follow out the consistent view, to bring the truth to light. What, then, is to be said about these theories? This that those who would have it that the soul migrates into natures divergent from each other seem to me to obliterate all natural distinctions; to blend and confuse together, in every possible respect, the rational, the irrational, the sentient, and the insensate; if, that is, all these are to pass into each other, with no distinct natural order secluding them from mutual transition.
To say that one and the same soul, on account of a particular environment of body, is at one time a rational and intellectual soul, and that then it is caverned along with the reptiles, or herds with the birds, or is a beast of burden, or a carnivorous one, or swims in the deep; or even drops down to an insensate thing, so as to strike out roots or become a complete tree, producing buds on branches, and from those buds a flower, or a thorn, or a fruit edible or noxious— to say this, is nothing short of making all things the same and believing that one single nature runs through all beings; that there is a connection between them which blends and confuses hopelessly all the marks by which one could be distinguished from another.
The philosopher who asserts that the same thing may be born in anything intends no less than that all things are to be one; when the observed differences in things are for him no obstacle to mixing together things which are utterly incongruous. He makes it necessary that, even when one sees one of the creatures that are venom-darting or carnivorous, one should regard it, in spite of appearances, as of the same tribe, nay even of the same family, as oneself. With such beliefs a man will look even upon hemlock as not alien to his own nature, detecting, as he does, humanity in the plant.
The grape-bunch itself, produced though it be by cultivation for the purpose of sustaining life, he will not regard without suspicion; for it too comes from a plant: and we find even the fruit of the ears of grain upon which we live are plants; how, then, can one put in the sickle to cut them down; and how can one squeeze the bunch, or pull up the thistle from the field, or gather flowers, or hunt birds, or set fire to the logs of the funeral pyre: it being all the while uncertain whether we are not laying violent hands on kinsmen, or ancestors, or fellow-country-men, and whether it is not through the medium of some body of theirs that the fire is being kindled, and the cup mixed, and the food prepared?
To think that in the case of any single one of these things a soul of a man has become a plant or animal, while no marks are stamped upon them to indicate what sort of plant or animal it is that has been a man, and what sort has sprung from other beginnings—such a conception as this will dispose him who has entertained it to feel an equal amount of interest in everything: he must perforce either harden himself against actual human beings who are in the land of the living, or, if his nature inclines him to love his kindred, he will feel alike towards every kind of life, whether he meet it in reptiles or in wild beasts.
Why, if the holder of such an opinion go into a thicket of trees, even then he will regard the trees as a crowd of men. What sort of life will his be, when he has to be tender towards everything on the ground of kinship, or else hardened towards mankind on account of his seeing no difference between them and the other creatures? From what has been already said, then, we must reject this theory: and there are many other considerations as well which on the grounds of mere consistency lead us away from it.
For I have heard persons who hold these opinions saying that whole nations of souls are hidden away somewhere in a realm of their own, living a life analogous to that of the embodied soul; but such is the fineness and buoyancy of their substance that they themselves' roll round along with the revolution of the universe; and that these souls, having individually lost their wings through some gravitation towards evil, become embodied; first this takes place in men; and after that, passing from a human life, owing to brutish affinities of their passions, they are reduced to the level of brutes; and, leaving that, drop down to this insensate life of pure nature which you have been hearing so much of; so that that inherently fine and buoyant thing that the soul is first becomes weighted and downward tending in consequence of some vice, and so migrates to a human body; then its reasoning powers are extinguished, and it goes on living in some brute; and then even this gift of sensation is withdrawn, and it changes into the insensate plant life; but after that mounts up again by the same gradations until it is restored to its place in heaven. Now this doctrine will at once be found, even after a very cursory survey, to have no coherency with itself. For, first, seeing that the soul is to be dragged down from its life in heaven, on account of evil there, to the condition of a tree, and is then from this point, on account of virtue exhibited there, to return to heaven, their theory will be unable to decide which is to have the preference, the life in heaven, or the life in the tree. A circle, in fact, of the same sequences will be perpetually traversed, where the soul, at whatever point it may be, has no resting-place. If it thus lapses from the disembodied state to the embodied, and thence to the insensate, and then springs back to the disembodied, an inextricable confusion of good and evil must result in the minds of those who thus teach. For the life in heaven will no more preserve its blessedness (since evil can touch heaven's denizens), than the life in trees will be devoid of virtue (since it is from this, they say, that the rebound of the soul towards the good begins, while from there it begins the evil life again). Secondly, seeing that the soul as it moves round in heaven is there entangled with evil and is in consequence dragged down to live in mere matter, from whence, however, it is lifted again into its residence on high, it follows that those philosophers establish the very contrary of their own views; they establish, namely, that the life in matter is the purgation of evil, while that undeviating revolution along with the stars is the foundation and cause of evil in every soul: if it is here that the soul by means of virtue grows its wing and then soars upwards, and there that those wings by reason of evil fall off, so that it descends and clings to this lower world and is commingled with the grossness of material nature. But the untenableness of this view does not stop even in this, namely, that it contains assertions diametrically opposed to each other. Beyond this, their fundamental conception itself cannot stand secure on every side. They say, for instance, that a heavenly nature is unchangeable. How then, can there be room for any weakness in the unchangeable? If, again, a lower nature is subject to infirmity, how in the midst of this infirmity can freedom from it be achieved? They attempt to amalgamate two things that can never be joined together: they descry strength in weakness, passionlessness in passion. But even to this last view they are not faithful throughout; for they bring home the soul from its material life to that very place whence they had exiled it because of evil there, as though the life in that place was quite safe and uncontaminated; apparently quite forgetting the fact that the soul was weighted with evil there, before it plunged down into this lower world. The blame thrown on the life here below, and the praise of the things in heaven, are thus interchanged and reversed; for that which was once blamed conducts in their opinion to the brighter life, while that which was taken for the better state gives an impulse to the soul's propensity to evil. Expel, therefore, from among the doctrines of the Faith all erroneous and shifting suppositions about such matters! We must not follow, either, as though they had bit the truth those who suppose that souls pass from women's bodies to live in men, or, reversely, that souls that have parted with men's bodies exist in women: or even if they only say that they pass from men into men, or from women into women. As for the former theory, not only has it been rejected for being shifting and illusory, and for landing us in opinions diametrically opposed to each other; but it must be rejected also because it is a godless theory, maintaining as it does that nothing among the things in nature is brought into existence without deriving its peculiar constitution from evil as its source. If, that is, neither men nor plants nor cattle can be born unless some soul from above has fallen into them, and if this fall is owing to some tendency to evil, then they evidently think that evil controls the creation of all beings. In some mysterious way, too, both events are to occur at once; the birth of the man in consequence of a marriage, and the fall of the soul (synchronizing as it must with the proceedings at that marriage). A greater absurdity even than this is involved: if, as is the fact, the large majority of the brute creation copulate in the spring, are we, then, to say that the spring brings it about that evil is engendered in the revolving world above, so that, at one and the same moment, there certain souls are impregnated with evil and so fall, and here certain brutes conceive? And what are we to say about the husbandman who sets the vine-shoots in the soil? How does his hand manage to have covered in a human soul along with the plant, and how does the moulting of wings last simultaneously with his employment in planting? The same absurdity, it is to be observed, exists in the other of the two theories as well; in the direction, I mean, of thinking that the soul must be anxious about the intercourses of those living in wedlock, and must be on the look-out for the times of bringing forth, in order that it may insinuate itself into the bodies then produced. Supposing the man refuses the union, or the woman keeps herself clear of the necessity of becoming a mother, will evil then fail to weigh down that particular soul? Will it be marriage, in consequence, that sounds up above the first note of evil in the soul, or will this reversed state invade the soul quite independently of any marriage? But then, in this last case, the soul will have to wander about in the interval like a houseless vagabond, lapsed as it has from its heavenly surroundings, and yet, as it may happen in some cases, still without a body to receive it. But how, after that, can they imagine that the Deity exercises any superintendence over the world, referring as they do the beginnings of human lives to this casual and meaningless descent of a soul. For all that follows must necessarily accord with the beginning; and so, if a life begins in consequence of a chance accident, the whole course of it becomes at once a chapter of accidents, and the attempt to make the whole world depend on a Divine power is absurd, when it is made by these men, who deny to the individualities in it a birth from the fiat of the Divine Will and refer the several origins of beings to encounters that come of evil, as though there could never have existed such a thing as a human life, unless a vice had struck, as it were, its leading note. If the beginning is like that, a sequel will most certainly be set in motion in accordance with that beginning. None would dare to maintain that what is fair can come out of what is foul, any more than from good can come its opposite. We expect fruit in accordance with the nature of the seed. Therefore this blind movement of chance is to rule the whole of life, and no Providence is any more to pervade the world.
Source: On the Soul and the Resurrection (New Advent)