9 But even as touching this I must not be ungrateful to my Master. I know that the human soul is spiritual not corporeal, that it is endowed with reason and intelligence, and that it is not of God's essence but a thing created. It is both mortal and immortal: the first because it is subject to corruption and separable from the life of God in which it is alone blessed, the second because its consciousness must ever continue and form the source of its happiness or woe.
It does not, it is true, owe its immersion in the flesh to acts done before the flesh; yet in man it is never without sin, not even when “its life has been but for one day.” Of those engendered of the seed of Adam no man is born without sin, and it is necessary even for babes to be born anew in Christ by the grace of regeneration. All this I know concerning the soul, and it is much; the greater part of it, indeed, is not only knowledge but matter of faith as well. I rejoice to have learned it all and I can truly say that I know it.
If there are things of which I am still ignorant (as whether God creates souls by generation or apart from it— for that He does create them I have no doubt) I would sooner know the truth than be ignorant of it. But so long as I cannot know it I had rather suspend my judgment than assert what is plainly contrary to an indisputable truth.
Source: Letters (New Advent)