Thomas Aquinas, Compendium of Theology

The assertion that a soul separated from its body can be tortured by corporeal fire should not seem nonsensical when we reflect that it is not contrary to the nature of a spiritual substance to be confined to a body. This happens in the ordinary course of nature, as we see in the union of the soul with the body. The same effect is sometimes produced by the arts of black magic, by which a spirit is imprisoned in images or amulets or other such objects. The power of God can undoubtedly bring it about that spiritual substances, which are raised above the material world by their nature, may nevertheless be tied down to certain bodies, such as hell-fire; not in the sense that they animate the body in question, but that they are in some way fettered to it. And this very fact, brought home to the consciousness of a spiritual substance, namely, that it is thus subjected to the dominion of a lowly creature, is grievous to it.

Inasmuch as this awareness is distressing to the spiritual substance, the contention that the soul "burns by the very fact that it perceives itself to be in fire," is substantiated. Thus understood, the fire is plainly spiritual, for what directly causes the distress is the fire apprehended as imprisoning. But inasmuch as the fire in which the spirit is incarcerated is corporeal fire, the further statement made by Gregory is borne out, namely, that "the soul is in agony not only because its perceives, but also because it experiences, the fire."

Furthermore, since this fire has the power of imprisoning the spiritual substance, not of its own nature, but by the might of God, the view is fittingly expressed by some that the fire acts on the soul as an instrument of God's vindictive justice. This does not mean that the fire acts on the spiritual substance as it acts on bodies, by heating, parching, and consuming; its action is restrictive, as we said. And since that which directly afflicts the spiritual substance is the awareness that the fire incarcerates it for its punishment, we can reasonably suppose that the suffering does not cease even if, by God's dispensation, the spiritual substance should happen for a time to be released from the fire. In the same way a criminal who has been sentenced to perpetual irons feels no dimunition of his unremitting pain even though the chains should be struck off for an hour.