Thanatology presupposes philosophical anthropology: what death is taken to be depends on what the human being is taken to be. Although Christianity certainly has affinities with Platonism, so much so that Nietzsche could with some justice speak of Christianity as Platonism for the people, the Christian view of man is in an important respect un-Platonic. In terms of Aquinas' Latin, Platonism holds that homo est anima utens corpore, man is a soul using a body. On this view the person is essentially the soul, and the body is a temporary and accidental housing or vehicle. There are Platonic passages in which the soul is described as "imprisoned" in the body. The body is the prison house of the soul. The soul is in the body like an oyster in its shell. These and other metaphors can be found attributed to Socrates in the Platonic dialogues. If one thinks in this way, then death is not a calamity but something good. Death is liberation, release, the separation of one thing, the soul, from another, the body, to which it should not have been attached in the first place. The fall into time is a fall into the flesh and the impermanence of the flesh. For Platonism, death undoes the fall into time. Death is to that extent good, and the philosopher welcomes it. Indeed, the philosophical life is a preparation for death. (Plato, Phaedo, 67e) The same holds for the Christian life.
Christianity, however, in terms of Aquinas' Latin, maintains that (a) homo non est anima tantum, man is not the soul alone, and (b) anima forma corporis, the soul is the form of the body. The human person, then is essentially a form-matter composite, and not essentially a soul. On the Platonic view soul can exist without body, though not conversely, while on the Christian view as understood by Aquinas, neither soul nor body can exist without the other.* (The footnote below nuances the point.) The two need each other: the body needs the soul to animate it and 'personalize' it, to make it a person's body as opposed to a corpse. And the soul needs the body to achieve its proper unfolding. Despite the difference between soul and body, man is a unitary being in a way in which he is not unitary on the Platonic substance-dualist conception.
There is another important difference. For Platonism, the fall into time — see Phaedrus — is a fall into an evil condition. But for Christianity, everything is and is good because it comes from God who is all-good. What the Creator creates cannot be evil. So our embodiment cannot be evil. For Christianity there is of course a Fall, but it is not a Fall into embodiment. It is a fall from a perfect form of embodiment into an imperfect form.
For Christianity, then, we are not immortal souls accidentally housed in mortal bodies, and death cannot be understood as affecting only our bodies. Death is not release of one part of the self from another, but the destruction of the entire unitary self. Being, bestowed as it is by God, is good, and so, if we are essentially embodied, death is a total calamity.
On the Christian view, then, we are not naturally immortal: we are mortal body and soul. Any immortality that we come to acquire requires a supernatural agent operating in a supernatural way. But weren't humans naturally immortal before the Fall? Didn't death enter the picture only when sin did?
It is true that on the Christian scheme death first enters the picture as punishment for sin. But it does not follow that we are naturally immortal in our pre-lapsarian state. The doctor angelicus discusses this question in the Summa Theologica, Q 97, art. 1. If pre-lapsarian, paradisiacal man were immortal by nature, then he could not have lost his immortality, which is precisely what happened when he sinned. Some of the angels sinned but did not lose their immortality because they are naturally immortal. But man is not: his pre-lapsarian immortality was a divine gift. By nature he is mortal, i.e., subject to death, able to die and unable to avoid dying. Aquinas speaks of a "supernatural force given by God to the soul, whereby it was enabled to preserve the body from all corruption so long as it itself remained subject to God."
In sum, man in his own nature is mortal, body and soul, in both his pre-lapsarian and post-lapsarian states. But owing to a supernatural gift, man in his pre-lapsarian state was given the power to preserve himself as long as he willed to so so. Pre-lapsarian man could die, but not against his will. We, however, are condemned to death nolens volens by Adam's sin.
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*On Aristotelian hylomorphic metaphysics, the form and the matter of a primary sublunary substance are not themselves substances: they cannot exist on their own apart from the substance of which they are 'principles' or ontological factors. In particular, the soul, as the substantial form of an animal body, cannot exist disembodied. But then what accounts for the continuity of the human person from the time of death to the resurrection of the body? As I understand Aquinas, he simply makes an exception in the case of the deceased human animal: in this case the soul as form can exist apart from a material substratum. A Platonic element is retained within the broadly Aristotelian schema. This exception goes along with a second exception in the case of God himself. God is forma formarum, form of all forms. God is self-subsistent pure form, form without matter. The human soul between death and resurrection is a pure form as well, although not self-subsistent pure form.
The first exception will strike some philosophers as an objectionably ad hoc move that does not fit within Aristotelian hylomorphism. The move is motivated by the need to accommodate the Christian revelation within the Aristotelian metaphysical scheme. If the forms of sublunary substances are not themselves substances, then this should hold across the board and allow of no exceptions. Or so one might think. The second exception seems equally ad hoc. If forms are principles, and not substances in their own right, then then they cannot exist on their own apartfrom what they inform. Hence there cannot be any self-subsistent pure forms.
It has been said that Aquinas is a Aristotelian on earth, but a Platonist in heaven. Is this an insurmountable problem? This is equivalent to asking whether the ad hoc point made above amounts to a solid objection. One could argue that God as Being itself, ipsum esse subsistens, is unique, and indeed uniquely unique, and therefore cannot be placed within an ontological scheme applicable to beings, entia. If God is esse, then he is Being itself, not a being among beings. If so, there is nthing objectionably adhoc about making the second exception above. As for the first, its evaluation is less clear. I’ll leave it at that for today.