The philosophy of the soul is certainly an interesting one. Below are some of the “options” that folks have come up with in the history of the Church.

The Universal Soul
Aristotle as interpreted by the Muslim philosopher Averroes said that there was but one intellectual soul in the universe and that we all participated and shared this soul. Muslims were not keen on this and neither were Christians. Saint Thomas Aquinas put the smack down on this. This view also rears its head in the likes of Scotus Eriugena.

Tripartite Soul
There is the Gnostic doctrine that the soul is tripartite: earthly (fleshly), soulish, and spiritual. This view is for a large part taken from misunderstandings of Saint Paul’s writings–especially 1 Corinthians. Gnostics later classified people according to this tripartite characteristics: the spiritual were beyond ethical norms. The soulish were your run of the mill Christians of the pew. And the fleshly were the unconverted.

Pre-Existent Soul
Then there is the error that the soul is pre-existent (even eternal) and is placed into the body. (Some Gnostics taught this with respect to the privileged souls of the spiritual ones.) Origen seemed to teach that all souls pre-existed creation and this doctrine is certainly found in the canonical book of the Wisdom of Solomon:

“As a child I was by nature well endowed, and a good soul fell to my lot; or rather, being good, I entered an undefiled body. But I perceived that I would not possess wisdom unless God gave her to me.”

Wisdom 8:19-21

Quasi-Material Soul and Traducianism
Then there is the Traducianism of Tertullian, who taught that the soul was a quasi-material substance that was transmitted form parents to child. It was like “soul DNA”. A part of mommy’s soul blended with a part of daddy’s soul and made a blended “baby soul” that formed along with the “baby body” that also derived from the two parents. Saint Augustine is alleged to have taught this, but others have made a case that he subscribed to Creationism (see below). You can see how it would explain the transmission of original sin from soul to soul. Incidentally, Saint Jerome, who was aware of the Eastern tendency to posit the pre-existence of souls, stated that Traducianism was the common theory in the West.

Saint Thomas Aquinas and the Scholastics on the Soul
Thomas Aquinas taught that intellectual soul (which included the sensitive and vegetative principles) is the form of the body. The problem with Saint Thomas is they way in which he understands the soul as it relates to human conception. At conception, the embryo develops the principles of life merely by vegetative powers. Next, the sensitive soul emerges. Finally, the intellectual soul quickens the embryonic body and completes the baby so that it becomes a human person. Obviously, this doctrine doesn’t square with modern science – though I can see some crafty pro-choice folks picking it up and running with it.

Creationism and the Soul
The Catholic Church teaches “creationism” with regard to the soul. The soul is created ex nihilo at the moment of natural conception. The soul does not pre-exist, nor does it derive from the souls of the parents.

It would appear that God creates the soul immediately at conception. We know this because Mary is the Immaculate Conception (she was conceived without original sin) and so she was morally immaculate at her conception. Consequently, she must have been a moral agent at conception and thus she possessed her soul at the moment of conception.

In summary, the soul is created at conception. At death it continues to have consciousness. At the Last Judgment the soul reunites with the body. It is everlasting, but it is not eternal, in the strictest sense of the words.