Who knows if the life breath of mortals goes upward and the life breath of beasts goes earthward? (Eccl. 3:21)
For the living know that they are to die, but the dead no longer know anything. There is no further recompense for them, because all memory of them is lost."(Eccl. 9:5)
the dust returns to the earth as it once was,
Ecclesiastes seems to indicate a certain uncertainty about life after death but this is only from the world as seen by author's senses. The author of Ecclesiastes uses the above expressions to reveal the vanity of things in this world because with death human achievement passes away. The dead are no longer remembered or given reward for their labors. The verses above neither prove or disprove the immorality of the soul. He only knows that the soul or the "life breath" returns to God.
For certainty of the immortality of the soul we must look to Christ who points out that at the Burning Bush God said that he Is the God of Abraham, Isaac and Jacob, "He is not the God of the dead but of the living. (Mt 22:32) In the parable of the Rich Man and Lazarus we read, "When the poor man died, he was carried away by angels to the bosom of Abraham. The rich man also died and was buried, and from the netherworld, where he was in torment, he raised his eyes and saw Abraham far off and Lazarus at his side. And he cried out, Father Abraham, have pity on me. Send Lazarus to dip the tip of his finger in water and cool my tongue, for I am suffering torment in these flames."(Lk 16:22-24) Both Lazarus and the Rich Man were alive after death which indicates the immortality of the soul. Christ further specifies the type of life we will live, "At the resurrection they neither marry nor are given in marriage but are like the angels in heaven." ( 22:30) In God loving plan for us life is not taken away but is transformed in a marvelous way.
we show the soul's immortality by using reasons alone?
Reason 1. SOUL IS A SIMPLE SUBSTANCE
The soul is a simple substance which means it has no parts. It is a vivifying and unifying spiritual principle of my being. The body dies because it is made up of part but my soul cannot die. You can rightly object that plants and animals have souls yet we do not consider them immortal. St. Thomas Aquinas states that the souls of plants and animals are derived from the potency of matter, so when they die their souls cease to exist. But, how do we know that they are derived from matter? Because the operations of the plant and animal soul is only directed to survive in time. Since they do not have an intellectual soul, they cannot desire to live forever.
Reason 2. MAN'S DESIRES TO LIVE FOREVER
Why would God give man the desire to live forever if he was not meant to life forever? Aristotle tells us that "A potency without possibility destroys nature." For example, an acorn has a potency of become an oak tree, but if no acorn ever became an oak tree, there would be no oak trees. This clearly illustrates that potency with possibility is in the nature of things.
Reason 3. MAN'S SOUL FORMS IDEAS OF REALITIES WHICH ARE IMMATERIAL
We have formed in our mind concepts. For example, we have a universal and immaterial concept of a tree in our minds, regardless of its shape, size and color. We have seen may kinds of trees such as the pine, birch and willow, but our immaterial tree does not belong to any of these trees. We can do this because our soul is spiritual or immaterial. No animal has a concept of an abstract tree. Animals have only images of actual trees. Further, the will has the power to strive after immaterial realities such as truth and justice which spring from the immortal principle (the soul) that can will these spiritual realities.
Reason 4. THE TESTIMONY OF MANKIND
Every culture in history has given testimony to its belief in an afterlife. Egypt especially has left us great monuments and inscriptions to its belief. In the above we see Henefer is led by Anubis, god of embalming, to the scales where his heart is weighed to see if it is worthy to proceed to paradise. Belief in the afterlife has taken varied expression in history, from belief in the underworld, reincarnation, heaven or hell and Christianity belief in the resurrection of the body.
Reason 5. MORAL LAW PRESUPPOSES IMMORTALITY
Moral law is rooted in the justice and wisdom of God. Since final justice often does not occur in this life, so there must be a next life in which justice will be done, therefore, our souls must be immortal. (strangenotions.com/Seven Proofs for the Natural Immortality of the Soul)
If you do not believe that the human soul is immortal, think of this--
"It is better to believe that there is life after death, because if in fact there is a life after death, we shall be rewarded for our faith, and yet loose nothing if we are wrong. ... If we do not believe in life after death, and we are wrong, we will be punished by God." (Immortality/Internet Encyclopedia of Philosophy)