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The Roman Catholic Church teaches that a human is an inextricable composite of a body and soul; the human soul begins to exist when the body does.
This is different from the view of the reincarnationist Platonists, who think human souls exist before the body and that man is a βsoul clothed with a body.β
St. Thomas Aquinas, in Summa Contra Gentiles II cap. 83, gives some philosophical reasons against reincarnation.
Moreover, with the approval of the said council, we reject as erroneous and contrary to the truth of the catholic faith every doctrine or proposition rashly asserting that the substance of the rational or intellectual soul is not of itself and essentially the form of the human body, or casting doubt on this matter. In order that all may know the truth of the faith in its purity and all error may be excluded, we define that anyone who presumes henceforth to assert defend or hold stubbornly that the rational or intellectual soul is not the form of the human body of itself and essentially, is to be considered a heretic.
(cf. this answer to "How are the soul and the body 'related' to each other according to the RCC?")