Reincarnation exerts a real force of seduction on Western minds. After a general presentation in the first article, the second article gave the judgments of the Church. The third and fourth articles presented the points of conflict between metempsychosis and Catholic dogma. The following articles examine the question from the point of view of philosophy.
The confrontation between the theory of reincarnation and the Catholic faith has shown their radical opposition. It remains to illuminate the subject in the light of natural reason. The first point of view being that of the faith, the principal argument was the authority of God. Here intelligence seeks the principles of nature. It must therefore penetrate the very heart of things, which goes beyond the sensible order.
What are the philosophical presuppositions and the difficulties raised by metempsychosis?
If the soul must go through several earthly lifetimes before attaining bliss, and pass from body to body, then it is not particularly tied to any of them. The soul is in the body only occasionally, so it remains foreign to it. This supposes a special conception of the soul and its relationship with the body.
In addition, metempsychosis, which admits reincarnation in beings other than humans, seems to grant the souls of plants and animals the same prerogatives as the human soul.
Some proponents of this thesis maintain that they still remember their past lives. Here is the problem of memory. Does it reside in the spiritual soul alone or in the body? In the latter case, shouldn't a change of body erase all memory of the past?
It must therefore be sequentially the soul in itself, then its relations with the body, and, finally, the powers of the soul, in particular the memory. 
The Soul in Itself
The soul presents itself to us under different aspects which must be studied separately although they are not separated in reality. The soul is the principle of life; it is the form of the body; it is the act of it.
The principle of life
The first experience that our senses allow, after that of the existence of things, is that of their movement. Observing the various movements separates the world into two distinct parts. Some beings only move under the action of an external principle. The others, on the contrary, have in themselves the principle of their movement. They move themselves.
This difference distinguishes living things from non-living things. “What distinguishes the living from the non-living is what life manifests in the first place and what remains until the end. However, the first thing that makes us say that an animal lives is that it begins to move, and it is said that it lives as long as this movement appears in it.” 
But the movement which translates life is only that which the thing gives to itself. “When he no longer moves by himself, but is only moved by another, then we say that the animal is dead, life has left him.”  What are the principal movements that present themselves to us?
Local movement: sand dunes move and change shape, but this is only due to the action of the wind. They are inert by themselves. On the contrary, it is through an inner dynamism that the fly flies and the dog runs.
The increase: ice stalactites grow larger, but only by addition. Their growth is only an accumulation of matter, while the moss on the roof grows by itself.
The grass in the garden grows by a phenomenon that cannot be explained only by external influences. But metal does not expand unless it is exposed to a heat source. If minerals developed on their own, we would have diamonds, silver, and gold galore!
The other kinds of movements proper to living things, nutrition and generation, lead us to the same results. The living being is one who moves by himself, due to an internal dynamism which is not reduced to external actions, what the philosophers have summed up in a concise definition: life is movement by itself.
* The principle of movement by oneself
What is, in the very nature of a living being, what gives it to move by itself, and so radically distinguishes it from the non-living? What is the principle of this “movement by oneself”?
Common language provides an indication: “We say of the living that they are animate, and of those that have no life that they are inanimate.”  It is thus the fact of being “animated,” of having a soul, that makes a thing considered to be alive. To be alive is to have a soul.
This fact is confirmed by one observation: different soul, different activity. The animal, for example, moves by its own movement, unlike plants. “Sensible perception is likewise a certain change; but it is only found in those who have a soul. Likewise the movement of growth and decrease is only found in those who feed themselves. But only those who have a soul feed themselves. It is therefore the soul that is the principle of all these movements.”  This is the first definition of the soul: the soul is the principle of all these movements.
The soul is therefore the vital function of a living body. It is a source of life and movement. The etymology underlines it well: the Latin anima translates the Greek Ψυχε which comes from the verb Ψυχω, I breathe. Which means that the soul is in immediate relation with a body; its very function is to be the source of life for a body. A soul cannot be conceived without its correlative, the body it vivifies.
On the contrary, the followers of reincarnation imagine the soul as created for itself, justifying itself on its own and not in a necessary relationship with a body. The union of soul and body is only the result of a fault, no longer a natural state.
On the other hand, if the soul is by its nature the principle of life of a body, it means that it is not life itself, but one of its parts. What lives is not the soul alone, but the composite body and soul. We “do not say that the soul walks, sees or hears, because it is man who does it through it [the body]. It is not the soul that exercises, by itself, any of the vital functions, but it is the animated being that exercises them through the soul.” 
This is the observation of common experience. Consider a man who walks, smells a flower, remembers an event, thinks about the future, and begins to pray. What is the subject of all these operations? Is it, successively and without link between them, each of its powers: its motor faculty, its sense of smell, its memory, and its mind? Is it only his body, then only his soul?
Is it not rather the same person, the composite body and soul? We all have a sense of this unity in our lives and we express it each time we use the pronoun “I.” It is one and the same “I” who sleeps, eats, dreams, or regrets his faults.
For reincarnation, on the contrary, the soul alone is the living one. The body is for the soul only an occasional and interchangeable abode.
Fr. Jean-Dominique, OP