I wanted to add a couple of things which might help clarify what I said in my first post.
Here is St. Thomas and his demonstration for the incorruptibility of the soul. This is from the Summa Theologica, Part 1, Question 75, Article 6
I answer that, We must assert that the intellectual principle which we call the human soul is incorruptible. For a thing may be corrupted in two ways--"per se," and accidentally. Now it is impossible for any substance to be generated or corrupted accidentally, that is, by the generation or corruption of something else. For generation and corruption belong to a thing, just as existence belongs to it, which is acquired by generation and lost by corruption. Therefore, whatever has existence "per se" cannot be generated or corrupted except 'per se'; while things which do not subsist, such as accidents and material forms, acquire existence or lost it through the generation or corruption of composite things. Now it was shown above (2,3) that the souls of brutes are not self-subsistent, whereas the human soul is; so that the souls of brutes are corrupted, when their bodies are corrupted; while the human soul could not be corrupted unless it were corrupted "per se." This, indeed, is impossible, not only as regards the human soul, but also as regards anything subsistent that is a form alone. For it is clear that what belongs to a thing by virtue of itself is inseparable from it; but existence belongs to a form, which is an act, by virtue of itself. Wherefore matter acquires actual existence as it acquires the form; while it is corrupted so far as the form is separated from it. But it is impossible for a form to be separated from itself; and therefore it is impossible for a subsistent form to cease to exist.
Granted even that the soul is composed of matter and form, as some pretend, we should nevertheless have to maintain that it is incorruptible. For corruption is found only where there is contrariety; since generation and corruption are from contraries and into contraries. Wherefore the heavenly bodies, since they have no matter subject to contrariety, are incorruptible. Now there can be no contrariety in the intellectual soul; for it receives according to the manner of its existence, and those things which it receives are without contrariety; for the notions even of contraries are not themselves contrary, since contraries belong to the same knowledge. Therefore it is impossible for the intellectual soul to be corruptible. Moreover we may take a sign of this from the fact that everything naturally aspires to existence after its own manner. Now, in things that have knowledge, desire ensues upon knowledge. The senses indeed do not know existence, except under the conditions of "here" and "now," whereas the intellect apprehends existence absolutely, and for all time; so that everything that has an intellect naturally desires always to exist. But a natural desire cannot be in vain. Therefore every intellectual substance is incorruptible.
This whole question should be studied, as it gives a closely reasoned demonstration for the nature of the soul and how it is related to the body and of its powars. See here:
Now the question is, does incorruptibility mean eternal? I know reputable Thomistic scholars who say Thomas does not say the soul is eternal, only that it is incorruptible. I agree. Though I think the rational evidence is in favor of an eternal soul, for this reason. Thomas demonstrated the existence of a first and final cause of everything which he showed to have many of the attributes of the Being we call God. But pretend for a moment that there had been no Revealation. We would still know that there existed a Being that was the Cause of all things, was Eternal, Intelligent, all Good, etc. Given the existence of such a Being, why would he create us if our end was oblivion.?It seems to me and many others that this would be contrary to the nature of such a Being. This is not proof but it does show where the evidence lays. And of course the eternity of the soul can be known for certain only as a fact of Divine Revealation. If you want absolute certainty, you will arrive at Revealation, there is no other way.