Is there anything in this world which forms as it were a world apart, a being that is wholly distinct from the rest of the world created by God?
Yes; and this being is man.
What is man?
Man is a composite of spirit and body, in whom the world of spirits and the world of bodies in some sort coalesce (LXXV.).
What is the spirit called that is in man?
It is called the soul (LXXV. 1-4).
Is man the only being in the world of bodies that has a soul?
No. Besides man plants and animals have souls.
What is the difference between the soul of man and the souls of plants and animals?
There is this difference, the soul of a plant has only vegetative life, the soul of an animal has both vegetative. and sensitive life, whereas the soul of man has in addition an intellective life.
Is it then by intellective life that man is distinct from all other living beings in this world?
Is this intellective life of the soul of man, in itself, independent of his body?
Yes (LXXV. 2).
Can any reason be given to establish this truth?
Yes; and the reason is because the object of thought is something wholly immaterial.
But how does it follow from this that the human soul in its intellective life is, in itself, independent of body?
This follows because if the soul itself were not wholly immaterial it could not attain by thought to an object wholly immaterial (ibid.).
What follows from this truth?
It follows that the soul of man is immortal (LXXV. 6).
(Next - The Catechism of the Summa - XIII. OF MAN: HIS NATURE; HIS SPIRITUAL AND IMMORTAL SOUL (B))
Pegues OP, R P Thomas. Catechism of the "Summa Theologica" of Saint Thomas Aquinas for the Use of the Faithful (pp. 62-64). Veritatis Splendor Publications. Kindle Edition.