The difficulty does not consist in proving that a spiritual principle exists within us, that is to say, a principle which resists death, because, in order to contest the existence of this principle, it would be necessary to contest thought. The real problem is to find out whether this spiritual and immortal principle which we bear within us, is to live again, after our death, in ourselves or in others. The question is, whether the immortal soul will be born again in the same individual, physically transformed, in the same person, in the ego, or whether it will pass into the possession of a being strange to that person.
We may remark here that on this all the interest of the question for us turns. It would be of very little importance to us, in reality, whether the soul were immortal or not, if the soul of each of us, being really indestructible and immortal, should pass to another than ourselves, or if, reviving in us, it did not possess the memory of our past existence. The resurrection of the soul without the memory of the past would be a real annihilation, this would be the nothingness of the materialists. It must be, then, that the soul lives again after our death, in ourselves, and that this soul, then, has clear remembrance of all the actions which took place in its previous existences. It behoves us, in short, to know, not whether our souls are immortal—that fact is self-evident—but whether they will belong to us in the other life, whether, after our death, we shall have identity, individuality, personality. It is to the study of this question that the present work is devoted. We are endeavouring to prove that the soul of the man remains always the same, in spite of its numerous peregrinations, notwithstanding the variety of form of the bodies in which it is successively lodged, when it passes from the animal to the man, from the man to the superhuman being, and from the superhuman being, after other celestial transmigrations, to the spiritualized being who inhabits the sun. We are endeavouring to establish that the soul, notwithstanding all its journeys, in the midst of its incarnations and various metamorphoses, remains always identical with itself, doing nothing more in each metempsychosis, in each metamorphosis of the exterior being, than perfect and purify itself, growing in power and in intellectual grasp. We are endeavouring to prove, that, notwithstanding the shadows of death, our individuality is never destroyed, and that we shall be born again in the heavens, with the same moral personality which was ours here below; in other words, that the human person is imperishable. It is for the reader to say whether we have attained our object, whether we have established the truth of this doctrine conformably with the laws of reasoning and the facts of science.
If an absolute demonstration of the existence of an immaterial principle in us be insisted upon, we must reply, that philosophy, like geometry, has its axioms, that is to say, its self-evident truths, which need not, or, if we choose to say so, which cannot be mathematically demonstrated. The existence of the soul is one of those axioms of philosophy. Diogenes answered a rhetorician who denied movement by walking in his presence. By expressing any thought, by saying "yes," or "no," we may prove the existence of the immortal soul to the sophists who would attempt to contest it.
We have just said that geometry has its axioms. Let us remember that an entire school of geometricians amused themselves by disputing the axioms, under the pretext that it was impossible to demonstrate them. We were present, in December, 1866, at a curious sitting of the Institute, during which M. Lionville, a celebrated mathematician, and professor at the Sorbonne, explained this strange polemic with great skill.
In attempting to demonstrate the propositions of geometry, certain axioms, i.e., self-evident truths, must be admitted in the first place. Otherwise, the primary reasoning will have no basis. But, among the numerous propositions of this kind which present themselves to the mind, and which result from the admission of one of their number, which is the most evident? That depends on the nature of the mind of each of us, and therefore it is that there is not, and that there never will be, an argument on this question.
There is a school of geometry which pretends to demonstrate everything. There is another, the true and good school, which, recognizing that the human mind has limits, and that everything is not accessible by our thoughts, lays down, under the name of axioms, certain truths which do not require proof, or, which is often the same thing, are incapable of proof.
Among the number of self-evident truths, or truths difficult of demonstration, we find the question of parallel lines. What are two parallels? Two lines which never meet each other. But how can we prove this property of two lines by reasoning? That is not, exactly speaking, possible, since the notion of the infinite is not admitted, or not understood by everybody, and cannot, therefore, serve as the basis of an absolutely rigorous argument.
It was for this reason that Euclid, the founder of geometry in ancient times, laid down this truth as a simple axiom, requiring (hence the postulates of Euclid, from the Latin verb postulare, to demand), that the truth of this principle, which he acknowledged himself unable to prove by logical demonstration, should be granted.
A hundred geometricians, since Euclid, who renounced the attempt to demonstrate it, have tried to prove this theory of parallels, but not one has succeeded. It was on the occasion of a fresh attempt at demonstration by a mathematician in the provinces, that M. Lionville spoke before the Academy, to recall the principles almost unanimously professed by geometricians on this subject.
The question is, in reality, thoroughly understood; it is treated on all works on geometry, and has been for a long time a settled matter. But certain minds are tempted by the subtlety of certain subjects, and the question of the postulatum turns up periodically before the learned societies, as it does in the conversations between the teachers of mathematics.