In the Summa Contra Gentiles, Thomas Aquinas presents many arguments for the immortality of the soul. Here is one of them:
 A further argument. It is impossible for natural desire to be in vain, “since nature does nothing in vain.” But every intelligent being naturally desires to be forever; and to be forever not only in its species but also in the individual. This point is made clear as follows. Natural appetite is present in some things as the result of apprehension; the wolf naturally desires the killing of the animals on which it feeds, and man naturally desires happiness. But in some other things natural desires results without apprehension from the sole inclination of natural principles, and this inclination, in some, is called natural appetite; thus, a heavy body desires to be down. Now, in both ways there is in things a natural desire for being; and a sign of this is that not only things devoid of knowledge resist, according to the power of their natural principles, whatever is corruptive of them, but also things possessed of knowledge resist the same according to the mode of their knowledge. Hence, those things lacking knowledge, in whose principles there is a power of keeping themselves in existence forever so that they remain always the same numerically, naturally desire to exist everlastingly even in their numerical self-identity. But things whose principles have not the power to do this, but only the power of perpetuating their existence in the same species, also naturally desire to be perpetuated in this manner. Hence, this same difference must be found also in those things in which there is desire for being, together with knowledge, so that those things which have no knowledge of being except as now desire to be as now, but not to be always, because they do not apprehend everlasting being. Yet they desire the perpetual existence of the species, though without knowledge, because the generative power, which conduces to this effect, is a forerunner and not a subject of knowledge. Hence, those things which know and apprehend perpetual being desire it with natural desire. And this is true of all intelligent substances. Consequently, all intelligent substances, by their natural appetite, desire to be always. That they should cease to be is, therefore, impossible.
The argument as I understand it (with the help of this blog post by Edward Feser) goes roughly as follows:
- If something has a natural desire for something, then that desire cannot be in vain, i.e. it must in principle be possible to fulfill that desire.
- All things have a natural desire to exist.
- Humans have knowledge of eternal truths about existence.
- Thus humans have a natural desire to exist forever.
- Therefore, humans exist forever.
My question is, what is Aquinas' argument for step 4, i.e. for the notion that humans have a natural desire to exist forever?
His argument for step 4 is given in bold, but I'm not really able to understand what he's saying. First of all he says "Hence, those things lacking knowledge, in whose principles there is a power of keeping themselves in existence forever so that they remain always the same numerically, naturally desire to exist everlastingly even in their numerical self-identity. But things whose principles have not the power to do this, but only the power of perpetuating their existence in the same species, also naturally desire to be perpetuated in this manner." I don't know what "numerical self-identity" and "existence in the same species" are. But in any case, whatever this statement means, he seems to think that it implies that those who have knowledge of eternal existence have a natural desire for eternal existence, and those who only have knowledge of existence in the here and now have only a natural desire for existence in the here and now.
Can someone explain Aquinas' reasoning?