8. Everything that perishes, perishes by suffering something. Destruction is a sort of suffering. But no subsistent intelligence can suffer any impression such as to lead to its destruction. For to suffer is to receive something; and whatever is received in a subsistent intelligence must be received according to the manner of the same: that is to say, it must be received as an intelligible impression. But whatever is so received in a subsistent intelligence, goes to perfect that intelligence, not to destroy it: for the intelligible is the perfection of the intelligent. A subsistent intelligence therefore is indestructible.*
10. The intelligible is the proper perfection of the intellect: hence the understanding in the act of understanding, and its term, or object in the act of being understood, are one.* What therefore belongs to the object as intelligible, must belong also to the mind as cognisant of that object; because perfection and perfectible are of the same genus.* Now the intelligible object, as such, is necessary and imperishable: for things necessary, or things that must be, are perfectly cognisable to the understanding; while things contingent, that are but might not be, as such, are cognisable only imperfectly: they are not matter of science, but of opinion.* Hence the understanding attains to science of perishable things, only in so far as they are imperishable, -- that is to say, in so far as they become to the mind universals. Intellect therefore, as such, must be indestructible.*
13. It is impossible for a natural desire to be void of object, for nature does nothing in vain. But every intelligence naturally desires perpetuity of being, not only perpetuity of being in the species, but in the individual: which is thus shown. The natural desire which some creatures have arises from conscious apprehension: thus the wolf naturally desires the killing of the animals on which he feeds, and man naturally desires happiness. Other creatures, without any conscious apprehension, are led by the inclination of primitive physical tendencies, which is called in some 'physical appetite.' The natural desire of being is contained under both modes: the proof of which is that creatures devoid of any sort of cognitive faculty resist destructive agencies to the full strength of their natural constitution, while creatures possessed of any manner of cognitive faculty resist the same according to the mode of their cognition. Those creatures therefore, devoid of cognition, who have in their natural constitution strength enough to preserve perpetual being, so as to remain always the same numerically, have a natural appetite for perpetuity of being even in respect of sameness of number: while those whose natural constitution has not strength for this, but only for preservation of perpetuity of being in respect of sameness of species, also have a natural appetite for perpetuity. This difference then must be noted in those creatures whose desire of being is attended with cognition, that they who do not know being except in the present time, desire it for the present time, but not for ever, because they have no apprehension of everlasting existence: still they desire the perpetual being of their species, a desire unattended with cognition, because the generative power, which serves that end, is preliminary to and does not come under cognition. Those then that do know and apprehend perpetual being as such, desire the same with a natural desire. But this is the case with all subsistent intelligences. All such subsistent intelligences therefore have a natural desire of everlasting being. Therefore they cannot possibly cease to be.
13. All things that begin to be, and afterwards cease to be, have both their beginning and their ceasing from the same power: for the same is the power to make to be and to make not to be. But subsistent intelligences could not begin to be except through the power of the prime agent. Therefore neither is there any power to make them cease to be except in the prime agent, inasmuch as that agent may cease to pour being into them. But in respect of this power alone nothing can be called perishable; as well because things are called necessary or contingent in respect of the power that is in them, not in respect of the power of God (Chap. XXX), as also because God, the author of nature, does not withdraw from things that which is proper to their nature; and it has been shown that it is proper to intellectual natures to be perpetual.
2.53 : That in Created Subsistent Intelligences there is Actuality and Potentiality
2.56, 69 : How a Subsistent Intelligence may be United with a Body, with a Solution of the Arguments alleged to prove that a Subsistent Intelligence cannot be United with a Body as its Form