This is explained by a sentence at the end of the previous chapter: -- "The bodies of the damned shall be afflicted by corporeal fire, inasmuch as the keen quality of that fire is contrary to that equable bodily structure and harmony, which is connatural to sense, though it cannot break up the structure: for such affliction shall not be able to separate the soul from the body, since the body must ever remain under the same form." -- The notion is that the soul, reunited with the body in hell, has such a drastic hold upon the body that not one bodily atom can be torn from its place: on the other hand the fire too takes hold of the same body, and endeavours, as fire ever will do, to disintegrate and break the body up: thus over that body a conflict rages between the immortal soul and the everlasting fire. And this is the 'contrariety' in question; and in this contrariety, felt, the agony of hell-fire consists.

Of God and His Creatures: 4.90