Objection 1. It would seem that the blessed in heaven will not see the sufferings of the damned. For the damned are more cut off from the blessed than wayfarers. But the blessed do not see the deeds of wayfarers: wherefore a gloss on Isaiah 63:16, "Abraham hath not known us," says: "The dead, even the saints, know not what the living, even their own children, are doing" [St. Augustine, De cura pro mortuis xiii, xv]. Much less therefore do they see the sufferings of the damned.
Objection 2. Further, perfection of vision depends on the perfection of the visible object: wherefore the Philosopher says (Ethic. x, 4) that "the most perfect operation of the sense of sight is when the sense is most disposed with reference to the most beautiful of the objects which fall under the sight." Therefore, on the other hand, any deformity in the visible object redounds to the imperfection of the sight. But there will be no imperfection in the blessed. Therefore they will not see the sufferings of the damned wherein there is extreme deformity.
On the contrary, It is written (Isaiah 66:24): "They shall go out and see the carcasses of the men that have transgressed against Me"; and a gloss says: "The elect will go out by understanding or seeing manifestly, so that they may be urged the more to praise God."
I answer that, Nothing should be denied the blessed that belongs to the perfection of their beatitude. Now everything is known the more for being compared with its contrary, because when contraries are placed beside one another they become more conspicuous. Wherefore in order that the happiness of the saints may be more delightful to them and that they may render more copious thanks to God for it, they are allowed to see perfectly the sufferings of the damned.
Reply to Objection 1. This gloss speaks of what the departed saints are able to do by nature: for it is not necessary that they should know by natural knowledge all that happens to the living. But the saints in heaven know distinctly all that happens both to wayfarers and to the damned. Hence Gregory says (Moral. xii) that Job's words (14:21), "'Whether his children come to honour or dishonour, he shall not understand,' do not apply to the souls of the saints, because since they possess the glory of God within them, we cannot believe that external things are unknown to them." [Concerning this Reply, Cf. I, 89, 8].
Reply to Objection 2. Although the beauty of the thing seen conduces to the perfection of vision, there may be deformity of the thing seen without imperfection of vision: because the images of things whereby the soul knows contraries are not themselves contrary. Wherefore also God Who has most perfect knowledge sees all things, beautiful and deformed.
Source: Summa Theologica (New Advent)