Allan Kardec's well-known 19th century volume The Spirits' Book (full text online) collects answers given by a variety of trance mediums to philosophical and theological questions. It makes interesting reading even today.
One of the main points raised by the purported spirit communicators is that reincarnation is a fact, and that every soul must undergo a large number of incarnations on Earth or on other physical worlds.
Kardec mounts an extended defense of the "justice" of reincarnation, which - although dated in its treatment of race and culture - makes some valuable points. What follows is an excerpt. The full chapter (Chapter 5 of Book 2) can be read here.
Italics are in the original. Long paragraphs have been broken up for easier reading.
Let us, for the present, examine the matter from another point of view, entirely irrespective of any assumed declarations of spirits in regard to it. Let us put the latter entirely aside for the moment; let us suppose them to have made no statement whatever in regard to it; let us even suppose the very existence of spirits not to have been surmised. Placing ourselves a moment on neutral ground, and admitting, as equally possible, the hypotheses of the plurality and of the unity of corporeal existences, let us see which of these hypotheses is most in harmony with the dictates of reason and with the requirements of our own interest.
There are persons who reject the idea of reincarnation simply because they do not like it, declaring that their present existence has been quite enough for them, and that they have no wish to recommence a similar one. Of such persons we would merely inquire whether they suppose that God has consulted their wishes and opinions in regulating the universe?
Either the law of reincarnation exists, or it does not exist. If it exists, no matter how displeasing it may be to them, they will be compelled to submit to it; for God will not ask their permission to enforce it. It is as though a sick man should say, "I have suffered enough today; I do not chose to suffer tomorrow." No matter what may be his unwillingness to suffer, he will nevertheless be obliged to go on suffering, not only on the morrow, but day after day, until he is cured. In like manner, if it be their destiny to live again corporeally, they will thus live again, they will be reincarnated. In vain will they rebel against necessity, like a child refusing to go to school, or a condemned criminal refusing to go to prison. They will be compelled to submit to their fate, no matter how unwilling they may be to do so.
Such objections are too puerile to deserve a more serious examination. Let us say, however, for the consolation of those who urge them, that the spiritist doctrine of reincarnation is by no means so terrible as they imagine it to be; that the conditions of their next existence depend on themselves, and will be happy or unhappy according to the deeds done by them in this present life; and that they may even, by their action in this life, raise themselves above the danger of falling again into the mire of expiation.
We take it for granted that those whom we are addressing believe in some sort of future after death, and that they do not look forward either to annihilation or to a drowning of their soul in a universal whole, without individuality, like so many drops of rain in the ocean; which comes to much the same thing. But, if you believe in a future state of existence, you probably do not suppose that it will be the same for all; for, in that case, where would be the utility of doing right? Why should men place any restraint upon themselves? Why should they not satisfy all their passions, all their desires, even at the expense of the rest of the world, if the result is to be the same in all cases?
On the contrary, you no doubt believe that our future will be more or less happy according to what we have done in our present life; and you have doubtless the desire to be as happy as possible in the future to which you look forward, since it will be for all eternity! Do you, perchance, consider yourself to be one of the most excellent of those who have ever existed upon the earth, and therefore entitled to supreme felicity? No. You admit, then, that there are some who are better than you, and who have consequently a right to a higher place, although you do not deserve to be classed among the reprobate.
Place yourself, then, in thought, for a moment, in the medium condition which, according to your own admission, will properly be yours, and suppose that some one comes to you and says, "You suffer; you are not so happy as you might be; and meanwhile you see others in the enjoyment of unmixed happiness. Would you like to exchange your position for theirs?" "Undoubtedly, I should," you reply; "what must I do to bring about such a result?" "Something very simple; you have only to begin again what you have done badly, and try to do it better." Would you hesitate to accept the offer, even at the cost of several existences of trial?
Let us take another illustration, still more prosaic. Suppose that someone comes to a man who, though not in a state of absolute destitution, has to endure many privations through the smallness of his means, and says to him, "Here is an immense fortune, of which you may have the enjoyment, on condition that you work hard during one minute." The laziest of men, in response to such an offer, would say, without hesitation, "I am ready to work for one minute, for two minutes, for an hour, for a whole day if necessary! What is a day's labor in comparison with the certainty of ease and plenty for all the rest of my life?"
But what is the duration of a corporeal life in comparison with eternity? Less than a minute; less than a moment.
We sometimes hear people bring forward the following argument:-"God, who is sovereignly good, cannot impose upon man the hard necessity of recommencing a series of sorrows and tribulations." But would there be more kindness in condemning a man to perpetual suffering for a few moments of error than in giving him the means of repairing his faults?
"Two manufacturers had each a workman who might hope to become some day the partner of his employer. But it happened that both workmen made so very bad a use of their day that they merited dismissal. One of the manufacturers drove away his unfaithful workman, despite his supplications; and this workman, being unable to obtain any other employment, died of want. The other said to his workman-'You have wasted a day; you owe me compensation for the loss you have thus caused me. You have done your work badly; you owe me reparation for it. I give you leave to begin it over again. Try to do well, and I will keep you in my employ, and you may still aspire to the superior position which I had promised you."
Need we ask which of the manufacturers has shown himself to be the most humane? And would God, who is clemency itself, be more inexorable than a just and compassionate man? The idea that our fate is decided forever by a few years of trial, and notwithstanding the fact that it was not in our power to attain to perfection while we remained upon the earth, fills the mind with anguish; while the contrary idea is eminently consoling, for it leaves us hope.
Thus, without pronouncing for or against the plurality of existences, without admitting either hypothesis in preference to the other, we assert that, if the matter were left to our own choice, there is no one who would prefer incurring a sentence against which there should be no appeal. A philosopher has said that "if God did not exist, it would be necessary to invent Him for the happiness of the human race;" the same might be said in regard to the plurality of existences. But, as we have already remarked, God does not ask our permission in the establishment of providential ordering; He does not consult our preferences in the matter. Either the law of reincarnation exists, or it does not exist; let us see on which side is the balance of probabilities, considering the matter from another point of view, but still leaving out of sight all idea of any statements that have been made by spirits in regard to it, and examining the question merely as matter of philosophic inquiry.
If the law of reincarnation does not exist, we can have but one corporeal existence; and if our present corporeal life be our only one, the soul of each individual must have been created at the same time as his body; unless, indeed, we assume the anteriority of the soul, in which case we should have to inquire what was the state of the soul before its union with the body, and whether this state did not constitute an existence of some kind or other.
There is no middle ground. Either the soul existed before its union with the body, or it did not. If it existed, what was its condition? Was it possessed of self-consciousness? If not, its state must have been nearly equivalent to non-existence. If possessed of individuality, it must have been either progressive or stationary; in either case, what was its degree of advancement on uniting itself to the body?
If, on the contrary, it be assumed, according to the general belief, that the soul is born into existence at the same time as the body-or that, previous to the birth of the body, it possesses only negative faculties-we have to propose the following questions:
1. Why do souls manifest so great a diversity of aptitudes independently of the ideas acquired by education?
2. Whence comes the extra-normal aptitude for certain arts and sciences displayed by many children while still very young, although others remain in a state of inferiority, or of mediocrity, all their life?
3. Whence do some individuals derive the innate or intuitive ideas that are lacking in others?
4. Whence do some children derive the precocious instincts of vice or of virtue, the innate sentiments of dignity or of baseness, which often contrast so strikingly with the situation into which they are born?
5. Why is it that some men, independently of education, are more advanced than others?
6. Why is it that among the races which people the globe some are savage and others civilized? ...
What is the philosophy or the theosophy that can solve these problems? Either the souls of men are equal at their birth, or they are unequal. If they are equal, why these inequalities of aptitude? Will it be said that these inequalities depend on the corporeal organization of each child? But such a doctrine would be the most monstrous and the most immoral of hypotheses; for, in that case, man would be a mere machine, the sport of matter; he would not be responsible for his actions, but would have the right to throw all the blame of his wrongdoing on the imperfections of his physical frame. If, on the other hand, souls are created unequal, God must have created them so; but, in that case, why is this innate superiority accorded to some and denied to others? And would such partiality be consistent with the justice of God, and the equal love He bears to all His creatures?
Admit, on the contrary, a succession of existences, and everything is explained. Men bring with them, at their birth in flesh, the amount of intuition they have previously acquired. They are more or less advanced, according to the number of existences they have previously accomplished, according as they are nearer to or farther from the common starting-point; exactly as, in a company made up of individuals of different ages, each will possess a degree of development proportionate to the number of years he has already lived; the succession of years being, to the life of the body, what the succession of existences is to the life of the soul....
God, in His justice, could not create souls more or less perfect. But granting the plurality of our corporeal existences, there is nothing in the differences of quality that we see around us in any way inconsistent with the most rigorous equity; for what we see around us is then perceived to have its roots, not in the present, but in the past....
We have [been] considering the soul in regard to its past and its present; if we consider it in regard to the future, we are met by difficulties which the theories in vogue are equally unable to explain:
1. If our future destiny is to be decided solely by our present existence, what will be in the future the respective positions of the savage and of the civilized man? Will they be on the same level, or will there be a difference in the sum of their eternal felicity?
2. Will the man who has labored diligently all his life to advance his moral and intellectual improvement be placed in the same rank with the man who, not through his own fault, but because he has had neither the time nor the opportunity for advancing, has remained at a lower point of moral and intellectual improvement?
3. Can the man who has done wrong because the means of enlightenment have been denied to him be justly punished for wrong-doing which has not been the result of his own choice?
4. We endeavor to enlighten, moralize, and civilize mankind; but, for one whom we are able to enlighten, there are millions who die every year without the light having reached them What is to be the fate of these millions? Are they to be treated as reprobates? and, if they are not to be so treated, how have they deserved to be placed in the same category with those who have become enlightened and moralized?
5. What is to be the fate of children who die before they have been able to do either good or evil? If they are to be received among the supremely happy, why should this favor be granted to them without their having done anything to deserve it? And in virtue of what privilege are they exempted from undergoing the tribulations of the earthly life?
Which of the doctrines hitherto propounded can solve these problems? But, if we admit the fact of our consecutive existences all these problems are solved in conformity with the divine justice. What we are not able to do in one existence we do in another. None are exempted from the action of the law of progress; every one is rewarded progressively, according to his deserts, but no one is excluded from the eventual attainment of the highest felicity, no matter what may be the obstacles he has to encounter on the road....
We have argued the matter, as we remarked above, without reference to statements made by spirits; such statements being, for many minds, without authority. If we, and so many others, have adopted the hypothesis of the plurality of existences, we have done so not merely because it has been proclaimed by spirits, but because it has appeared to us to be eminently rational, and because it solves problems that are insoluble by the opposite hypothesis. Had it been suggested to us by a mere mortal, we should, therefore, have adopted it with equal confidence, renouncing, with equal promptitude, our preconceived opinions on the subject; for when an opinion has been shown to be erroneous, even self-love has more to lose than to gain by persisting in holding it. In like manner, we should have rejected the doctrine of reincarnation, even though proclaimed by spirits, if it had appeared to us to be contrary to reason, as, indeed, we have rejected many other ideas which spirits have sought to inculcate, for we know, by experience, that we can no more give a blind acceptance to ideas put forth by spirits than we can to those put forth by men.
The principal merit of the doctrine of reincarnation is, then, to our minds, that it is supremely rational. But it has also in its favor the confirmation of facts-facts positive and, so to say, material, which are apparent to all who study the question with patience and perseverance, and in presence of which all doubt as to the reality of the law in question is impossible. When the appreciation of these facts shall have become popularized, like those which have revealed to us the formation and rotation of the earth, they who now oppose this doctrine will be compelled to renounce their opposition.
To sum up: We assert the doctrine of the plurality of existences is the only one which explains what, without this doctrine, is inexplicable; that it is at once eminently consolatory and strictly conformable with the most rigorous justice; and that it is the anchor of safety which God in His mercy has provided for mankind.
[Allan Kardec, The Spirits' Book, Ch. 5, Book 2]