The soul is the form of the body
The Hindu (and, I think, Platonic) idea of the soul is that it is just a passenger along for the ride in a bodily vehicle. Plato said, "Soma sema": The body (soma) is a prison (sema). Just as drivers can get out of a wrecked car and climb into a new one of any type, so Hindus imagine that the soul escapes from one dead body and can take possession of any other type of living being.
The Aristotelian, Thomist, and Catholic philosophical tradition sees the soul as the form of the body. Every living thing, even plants, have souls. It is the soul that causes one living being to be a plant, another to be an animal, and still another to be a human being. Souls are not interchangeable parts. A plant soul always produces a plant; an animal soul always produces an animal; a human soul always produces a human being.
- The soul may be defined as the ultimate internal principle by which we think, feel, and will, and by which our bodies are animated.
- St. Thomas's doctrine is briefly as follows:
- the rational soul, which is one with the sensitive and vegetative principle, is the form of the body. This was defined as of faith by the Council of Vienne of 1311;
- the soul is a substance, but an incomplete substance, i.e. it has a natural aptitude and exigency for existence in the body, in conjunction with which it makes up the substantial unity of human nature;
- though connaturally related to the body, it is itself absolutely simple, i.e. of an unextended and spiritual nature. It is not wholly immersed in matter, its higher operations being intrinsically independent of the organism;
- the rational soul is produced by special creation at the moment when the organism is sufficiently developed to receive it. In the first stage of embryonic development, the vital principle has merely vegetative powers; then a sensitive soul comes into being, educed from the evolving potencies of the organism — later yet, this is replaced by the perfect rational soul, which is essentially immaterial and so postulates a special creative act. Many modern theologians have abandoned this last point of St. Thomas's teaching, and maintain that a fully rational soul is infused into the embryo at the first moment of its existence.
The Hindu (and Harry Potter) view of the soul ("jiva") is that the soul is trapped inside a living organism and may migrate to other living organisms without losing its own self-identity. There is no intrinsic relationship between the soul and the body; any soul can inhabit any body; when the vehicle is destroyed, the soul is released to be re-incarnated in another form of life.
Incidentally, this shows what nonsense there is in the Harry Potter books about breaking souls into parts and preserving those parts alive in horcruxes. A body has parts, and we can live if we lose some parts of our bodies, but a soul is simple and indivisible. To live is to be a body informed (inwardly formed) by a soul; when the soul is separated from the body, the living being dies.
On these grounds, it is also nonsensical to think that the same human soul could animate other forms of life. My soul causes me to be me, both in this present body (somewhat worse for the wear) and in my resurrected body in Heaven.
Pious misinterpretations of "soul"
Because we are aware that God has given the human soul (the form of the body) spiritual powers that cause the soul to be immortal, and because we believe that the human soul can therefore continue to exist even when it is not forming a body, there is a lamentable tendency to lapse in Hindu forms of thought about our souls.
It is wrong to oppose the soul to the body, as if the soul exists apart from the body. The body without the soul is dead; the soul without the body is incomplete, for our soul is oriented toward the formation of a body. The soul is not at war with the body; our spiritual gifts (identity, intelligence, and free will; faith, hope, and love) are at odds with the tendencies of soul-formed-body, our animal life, with all of its disordered appetites, passions, and habits.
- The soul is not the antagonist of the body. Without the soul, there can be no body. Soul and body are a metaphysical unity.
- The body is not synonymous with the "flesh." The "flesh" is a spiritual tendency to sin, inherited among us as a consequence of Original Sin.
- Sin is a spiritual choice. Sin may involve misusing bodily powers, but the body is always innocent.