EVERY MAN HAS A SOUL
God is almighty and He knows everything. Indeed, all direction to a final end and all intelligence that exist in the cosmos come from Him.
Of all creatures, man is most equal to God, by his intellect. Indeed, human mind can imagine 'everything': he can assimilate it and in a sense become equal to it. By reasoning from nature, man even can determine some characteristics of God: like His incorporeity, His unchangeability and His timelessness. So human mind can climb to God! Saint Thomas deduces human mind is essentially incorporeal, although it needs the senses and the brain as long as it's tied to the body.
Man has also a bent to eternity: he wants eternal life. According to Saint Thomas, man has a 'natural longing for God'.
And God is such that He wants to answer this longing, because, if not, He wouldn't have laid it in man.
From these three considerations we can conclude human mind is immortal in principle. His final end is to behold God, as soon as he has risen above time and space. Hence it also follows that heaven exists.
Although we may translate the word 'soul' into the distinct languages with words that also denote concepts like 'wind', 'air', 'breath', the common meaning is always: the incorporeal and immortal essence of human mind, in connection with his consciousness. So the concept 'soul' is platonic. There is no direct relation with any part of the body, like the pineal gland, the brains or the heart.
I add some complementary remarks that thomistic philosophy teaches about the soul:
God created an ideal specimen of mine, which I have to become. Therefore my soul is my 'principle of form', that I'm striving after. On my own strength, I can't attain at this high perfection, but with the help of God's grace I can (in heaven).
We have the freedom to cooperate with this grace or to bear up against it. The latter is often easier, because the transient partial good is attractive, too, and it lies closer within our reach. But God respects our freedom. He wishes we give Him our love and our faith. It turns out this has much to do with turning to our fellow people and fellow creatures, just as Jesus showed it to us.