Catechism of the Summa (10)

 XIII. OF MAN: HIS NATURE; HIS SPIRITUAL AND IMMORTAL SOUL

(A)

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 

What is the spirit called that is in man?

It is called the soul 

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?

Yes

Is this intellective life of the soul of man, in itself, independent of his body?

Yes 

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 

What follows from this truth?

It follows that the soul of man is immortal 

(B)

Can it be shown that the immortality of man's soul follows from this truth?

Yes. Because if in the soul there is an act wholly independent of bodily matter, it must itself be independent of bodily matter.

What follows from this truth that the soul is, in itself, independent of bodily matter?

It follows that if the body perishes by separation from the soul, the soul itself does not perish 

Will the human soul live for ever?

Yes.

Why then is the human soul united to a body?

The human soul is united to a body in order to make a substantial whole called man 

Is it not then accidental that the soul is united to a body?

No, for the soul was made to be joined to a body 

What are the effects of the soul upon the body to which it is united?

The soul gives to the body every perfection that the body has, that is it gives to it being, life, and sense; but thought it cannot give, for this is proper to the soul itself