Objection 1. It would seem that the fire of hell whereby the bodies of the damned will be tormented will not be corporeal. For Damascene says (De Fide Orth. iv): The devil, and "demons, and his men" [Cf. 2 Thessalonians 2:3: "And the man of sin be revealed, the son of perdition."], namely Antichrist, "together with the ungodly and sinners will be cast into everlasting fire, not material fire, such as that which we have, but such as God knoweth." Now everything corporeal is material. Therefore the fire of hell will not be corporeal.
Objection 2. Further, the souls of the damned when severed from their bodies are cast into hell fire. But Augustine says (Gen. ad lit. xii, 32): "In my opinion the place to which the soul is committed after death is spiritual and not corporeal." Therefore, etc.
Objection 3. Further, corporeal fire in the mode of its action does not follow the mode of guilt in the person who is burned at the stake, rather does it follow the mode of humid and dry: for in the same corporeal fire we see both good and wicked suffer. But the fire of hell, in its mode of torture or action, follows the mode of guilt in the person punished; wherefore Gregory says (Dial. iv, 63): "There is indeed but one hell fire, but it does not torture all sinners equally. For each one will suffer as much pain according as his guilt deserves." Therefore this fire will not be corporeal.
On the contrary, He says (Dial. iv, 29): "I doubt not that the fire of hell is corporeal, since it is certain that bodies are tortured there."
Further, it is written (Wisdom 5:21): "The... world shall fight... against the unwise." But the whole world would not fight against the unwise if they were punished with a spiritual and not a corporeal punishment. Therefore they will be punished with a corporeal fire.
I answer that, There have been many opinions about the fire of hell. For some philosophers, as Avicenna, disbelieving in the resurrection, thought that the soul alone would be punished after death. And as they considered it impossible for the soul, being incorporeal, to be punished with a corporeal fire, they denied that the fire whereby the wicked are punished is corporeal, and pretended that all statements as to souls being punished in future after death by any corporeal means are to be taken metaphorically. For just as the joy and happiness of good souls will not be about any corporeal object, but about something spiritual, namely the attainment of their end, so will the torment of the wicked be merely spiritual, in that they will be grieved at being separated from their end, the desire whereof is in them by nature. Wherefore, just as all descriptions of the soul's delight after death that seem to denote bodily pleasure--for instance, that they are refreshed, that they smile, and so forth--must be taken metaphorically, so also are all such descriptions of the soul's suffering as seem to imply bodily punishment--for instance, that they burn in fire, or suffer from the stench, and so forth. For as spiritual pleasure and pain are unknown to the majority, these things need to be declared under the figure of corporeal pleasures and pains, in order that men may be moved the more to the desire or fear thereof. Since, however, in the punishment of the damned there will be not only pain of loss corresponding to the aversion that was in their sin, but also pain of sense corresponding to the conversion, it follows that it is not enough to hold the above manner of punishment. For this reason Avicenna himself (Met. ix) added another explanation, by saying that the souls of the wicked are punished after death, not by bodies but by images of bodies; just as in a dream it seems to a man that he is suffering various pains on account of such like images being in his imagination. Even Augustine seems to hold this kind of punishment (Gen. ad lit. xii, 32), as is clear from the text. But this would seem an unreasonable statement. For the imagination is a power that makes use of a bodily organ: so that it is impossible for such visions of the imagination to occur in the soul separated from the body, as in the soul of the dreamer. Wherefore Avicenna also that he might avoid this difficulty, said that the soul separated from the body uses as an organ some part of the heavenly body, to which the human body needs to be conformed, in order to be perfected by the rational soul, which is like the movers of the heavenly body--thus following somewhat the opinion of certain philosophers of old, who maintained that souls return to the stars that are their compeers. But this is absolutely absurd according to the Philosopher's teaching, since the soul uses a definite bodily organ, even as art uses definite instruments, so that it cannot pass from one body to another, as Pythagoras is stated (De Anima i, text. 53) to have maintained. As to the statement of Augustine we shall say below how it is to be answered (ad 2). However, whatever we may say of the fire that torments the separated souls, we must admit that the fire which will torment the bodies of the damned after the resurrection is corporeal, since one cannot fittingly apply a punishment to a body unless that punishment itself be bodily. Wherefore Gregory (Dial. iv) proves the fire of hell to be corporeal from the very fact that the wicked will be cast thither after the resurrection. Again Augustine, as quoted in the text of Sentent. iv, D, 44, clearly admits (De Civ. Dei xxi, 10) that the fire by which the bodies are tormented is corporeal. And this is the point at issue for the present. We have said elsewhere (70, 3) how the souls of the damned are punished by this corporeal fire.
Reply to Objection 1. Damascene does not absolutely deny that this fire is material, but that it is material as our fire, since it differs from ours in some of its properties. We may also reply that since that fire does not alter bodies as to their matter, but acts on them for their punishment by a kind of spiritual action, it is for this reason that it is stated not to be material, not as regards its substance, but as to its punitive effect on bodies and, still more, on souls.
Reply to Objection 2. The assertion of Augustine may be taken in this way, that the place whither souls are conveyed after death be described as incorporeal, in so far as the soul is there, not corporeally, i.e. as bodies are in a place, but in some other spiritual way, as angels are in a place. Or we may reply that Augustine is expressing an opinion without deciding the point, as he often does in those books.
Reply to Objection 3. That fire will be the instrument of Divine justice inflicting punishment. Now an instrument acts not only by its own power and in its own way, but also by the power of the principal agent, and as directed thereby. Wherefore although fire is not able, of its own power, to torture certain persons more or less, according to the measure of sin, it is able to do so nevertheless in so far as its action is regulated by the ordering of Divine justice: even so the fire of the furnace is regulated by the forethought of the smith, according as the effect of his art requires.
Source: Summa Theologica (New Advent)