It's not an unproved assumption - it follows from the definition of the soul as the form (forma) of the human being. You can substitute the words nature (natura) or substance (substantia or ousia) if you wish.
It is absolutely unproved as an experiential reality, precisely because it's based on abstract (non-experiential) definitions. You cannot intellectually prove the existence of the soul, period, your technical jargon notwithstanding.
Time is measured as the movements of bodies in space, but that does not mean time is the same as movements of bodies in space.
Imagine yourself in an environment with no space. No matter. No body. Nothing but your consciousness, just pure being -- and nothing (or no one) there with you either. Clearly there would be no time. Nothing aging (no body), nothing passing (nothing to pass), nothing arising (nothing to arise). Space-time is an indivisible matrix. If something does not exist in space, it does not exist in time either.
It's Kreeft commenting on Aquinas. What did you expect?
Who he is commenting on is irrelevant -- he could be commenting on Yogi Berra. The point is, he is passing on dogma, belief systems. In this case, Christian belief systems, yes, but all that matters is that people understand that they are belief systems.
In any case, Aquinas (and all Thomists) begin from Aristotle.
So what? What has that to do with anything? You're too impressed with so-called authority figures.
Once again, it follows from the definition of "soul" used by Aquinas.
And once again, though I'm beginning to doubt that you can get this point, you are talking about a definition. I am pointing toward that which is beyond "definitions".
And how do you know those insights/revelations are true? Or that they are true for other thinkers?
I'm not talking about "truth", and I'm not talking about "thinking". I'm talking about contemplative practice.
People can say all kinds of things about "what" the "soul" is, just like they can about the "taste of chicken". But I'm not referring to about "what" something is, in this case. I'm talking about direct experience -- that's what I'm referring to by "authority on the meaning of the soul".
For example, the Tao Te Ching, or the Upanishads, talk about the ultimate truth all the time. They (and other sacred texts) say all kinds of things about it. But none of this translates into direct experience, any more than a recipe book in itself translates into the direct experience of a chicken dinner.
Best analogy I know of about this whole conundrum is the Zen idea of the fingers pointing toward the Moon. The "fingers" represent words, ideas, systems, methods, sutras, texts, and so on. The "Moon" represents ultimate reality, or the soul, or God, etc.
The fingers direct one's attention toward the Moon, but they can't take us there. They are only pointers. Useful (or useless), but not to be misunderstood as the experiential truth itself.
What is really problematic is when we get attached to the fingers pointing toward the Moon. This is what happens when people simply pass on definitions of God, or soul, or spirit, etc., from religious doctrines -- "religious authority figures" -- and then assume it is the truth. That does't mean that these words in the texts are invalid, it just means that there is the real risk of mistaking them for the reality that they point toward. This is what it is to suffer under the effects of religious indoctrination. The word or name becomes a substitute for the spiritual experience.
In integral philosophy this is sometimes called a "category error". The idea there is that there are three basic realms of knowledge -- body, mind, spirit; or, physical, rational, contemplative. The soul, assuming it exists, belongs to the third category -- spirit/contemplation. To try to grasp it via the second category (mind/reason) is useless and will yield nothing, because it's a category error, same as it would be to apply something from the physical category (say, learning how to drive a car). You can no more understand "soul" via concepts about it read in a book, then you can via driving a car. "Soul" is only understood via contemplative practice (meditation, prayer, etc.).
This works the other way as well, of course. We can't understand how to drive a car (physical) or learn chess (mental) by sitting in meditation or praying to God (spiritual).
In religion, it's very common to try to apply reason/memory (mind) in attempting to grasp God (spirit). This is a category error and has led to profound problems all throughout history, especially when religious doctrines from different cultures appear to clash because of different mental interpretations of them. Many wars and much suffering have derived from these category errors.