Excerpts from the Dialogue between Socrates and Cebes in Plato's Phaedo:
What do we say about the soul, then? Is it visible or invisible?
Cebes: Not visible.
Invisible, then? Yes.
So soul is more like the invisible, and the body more like the visible?...
Then tell me, what must be present in a body to make it alive?
Is this always so? Of course.
So whenever soul takes possession of a body, it always brings life with it? Yes, it does.
Don't you think that it is the nature of the divine to rule and direct, and that
of the mortal to be subject and serve?
Cebes: I do.
Then which does the soul resemble?
Obviously, Socrates, soul resembles the divine, and body the mortal....
The human soul is the immediate creation of God, however, we are not to imagine it as if God is standing over conception ready to infuse the soul as a human would do. Karl Rahner explains it thus: "The origin of the soul supposes physical causes. But if in general the divine activity and the development of the world are intrinsically connected, transcendent and immanent casualties must merge where the world itself transcends itself absolutely. This self transcendence can be described as the creation of the soul, and here phylogenesis and ontogenesis are intrinsically connected." (Catholic Encyclopedia: Sacrementum Mundi, The Seabury Press, NY, 1975, p. 1618) To put it simply, the fertilized egg has in itself no ability to create the soul but if God gives it power to go beyond its physical limits, the intelligent soul becomes realized. This is a reasonable explanation keeping in mind that God brings about the soul from nothing.
How does the intelligent soul unite with the body?
The union of soul with the body has to be different from the union of material things by touch (bricks of a building), mixture such as water and sand or chemical change during which the matter becomes different. The intelligent soul has to inform the body without changing in itself. St. Thomas Aquinas long ago postulated a virtual union, that is, an activity of the intelligent soul reaching the deep recesses of matter and acting on all parts of the body simultaneously.
We understand this process better today with the discovery of genetic coding. The genetic code as information is in itself immaterial. The intelligent soul is able to interact and rearrange the genetic ancestral codes provided by the parents to form its own unique body. However, the human soul is much more than the organizing principle of the human body because its intelligence and free will makes the human being the image likeness of God. To state this another way, "If human bodies are, they are informed, and they must be informed by something that makes them human. This is the human soul, which at one and the same time gives existence to the body and specifies the nature of the composite. The human body can only be understood in terms of the kind of form actualizing it." (Essays.com/St. Thomas Aquinas) Finally, "The soul may be defined as the ultimate internal principle by which we think, feel, and will, and by which our bodies are animated." (On Line:New Avent, Catholic Encyclopedia/Soul) So where is the soul? St. Thomas Aquinas states that "Since the soul is united to the body as its form, it must necessarily be in the whole body, and in each part thereof. For it is not an accidental form, but the substantial form of the body. Now the substantial form perfects not only the whole, but each part of the whole." Thomas Aquinas, Summa Theologica, Part I, Q. 76.8)
The soul is non-material and invisible as such. It is not made up of parts. This is why when a man's leg is cut off, the soul still informs human nature. That the soul is spiritual can be observed in the intellect which is capable of abstract ideas. The picture of a tiger in our imagination is material and belongs to the body but the nature of a "cat", which includes all feline forms is a universal idea which is not restricted to any particular cat, therefore, it is not material. Further, since the soul is not composed of parts, it cannot die since death means a separation of parts of an organism. That DNA information which relates to the functioning of the human body disappears. What survives is the Person with its spiritual intellect and free will which now passes into another dimension of existence.
Of course, God could make the spiritual soul cease to exist at death but this is not his purpose in creating man. Christ confirmed the existence of the soul after death when he said that we would live like the angels of God. St. Paul refers to man's "spirit, soul and body". Spirit here means that from the creation man was ordered to a supernatural end and that his soul can be raised by God beyond his basic nature to communion with His divine life.
The soul by itself is not the complete person and can best be summarized in the word of the Catechism:
Man's soul transcends time and space and longs for the Eternal God. Please click Song image to hear.
Refrain: My soul is longing for your peace, near to you, my God.