Moreover we may take a sign of [the incorruptibility of the human soul] 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. (ST, I, 75, 6)
So, then, brute animals cannot conceive of eternal life, being locked into the here and now, and therefore cannot desire it. Humans, on the contrary, can imagine life everlasting and world without end, and naturally, as one, desire to live forever. And no natural desire is in vain, therefore…
Notice that my third objection to the C.S. Lewis’s version of this argument no longer applies in this case. For Lewis’s desire was clearly for heaven; it was a desire “which no experience in this world can satisfy.” Aquinas is far more modest here. He does not argue that there must be a perfect world for which we are meant; he simply says that humans desire to live forever, having the capacity to imagine and therefore to long for eternal life of whatever quality, heavenly or otherwise.