While I love Kreeft’s writings, there are arguments of his that I find very unconvincing and this is one of them (the Argument from Design is the other biggie that I dislike).
Self-awareness in itself can’t be the hallmark of the immortal soul in the Thomistic perspective, because self-awareness at the lowest level comes from sensing the self, which is a property of the “animal soul”. For more clarity, growing on your own through absorbing nutrients, and “having offspring” is the property of the “vegetative” soul; sensation and desire, and limited knowledge through these two avenues are properties of the “animal” or “sensitive” soul; the “rational” soul has intellect and free will, intellect being the thing by which we abstract and associate categories, reflect on deeper meanings, ect.
Now each of these souls contains the powers of the ones below it, so an animal soul contains the powers of the vegetative, and the rational soul contains the powers of both.
Since an animal can sense itself, and can tell that its senses don’t extend to those things outside of themselves, they can actually have a certain self-awareness. They can realize that they have their own desires, and feelings, and needs. We can see this in animal behavior all the time. The fact that certain animals like apes are able to use sign language to indicate their desires and feelings doesn’t mean they’re doing anything more, fundamentally speaking, than a bird does when it sings a mating call, or a dog does when it yelps after being stepped on: they have a desire or sensation, and they give a call based on it. Apes simply have more capable brains, and the means by which to make sign-language (human-like hands), so they’re able to extend this animal power in a way that others are not. Parrots, who can make sounds similar to human words are capable of the same feats but with voice.
What really sets humans apart is not self-awareness, but HOW we are self-aware. We are self-aware by sensation, but we are ALSO self-aware by intellect, meaning we can reflect on what it means to be me, the implications of my desires in the greater scheme of human society, we can wonder why I should be or desire at all, ect. All of those things rise above the senses; I can’t sense meaning, I can’t sense “human culture”, I can’t sense what it is that makes me sense (“you can’t feel your own finger with your finger”). So that Koko the gorilla signs that she’s feeling sad, or that she really wants a banana is actually nothing special, but rather to be expected. If she were to write poetry about sadness and bananas, or write an essay about how bananas are like sadness, or give a speech about what it means to be a gorilla in the modern world, then we would have indications of a rational soul. Those are things that the most limited functional humans can do, but the healthiest and smartest ape can’t ever hope to match.
In short, being self-aware of needs and feelings is animal; being self-aware of WHO I am, and WHAT I am, and WHY I am, that is a function of the rational immortal soul.
Hope that helps!
Peace and God bless!