Who said that the mere sight of devils was one of the greatest torments of hell? In Perelandra, chapter 9, C. S. Lewis writes:


  In certain old philosophers and poets he had read that the mere sight of the devils was one of the greatest among the torments of Hell.


Elsewhere in the Space Trilogy, Lewis makes specific references to John Milton and other real-world figures, so I doubt that this reference is fictional.  But he doesn't provide more detail regarding who said it.

Who might Lewis be referring to here?  Who originally said that the mere sight of devils in hell was one of its worst torments?
The most obvious reference I see comes from St. Catherine of Siena, one of the Doctors of the Catholic Church.
St. Catherine wrote a work known as the Dialogue, representing a discussion between a soul and God. In Section 22 of the Dialogue, God says this to the soul:

&quot;[The] vision of the Devil is the third pain [of four pains suffered by those in Hell] and redoubles to them their every toil.
&quot;As the saints exult in the sight of Me, refreshing themselves with joyousness in the fruit of their toils borne for Me with such abundance of love, and displeasure of themselves, so does the sight of the Devil revive these wretched ones to torments, because in seeing him they know themselves more, that is to say, they know that, by their own sin, they have made themselves worthy of him.
&quot;And if you remember well, you know that I showed him to you in his own form for a little space of time, hardly a moment, and you chose (after you had returned to yourself) rather to walk on a road of fire, even until the Day of Judgment, than to see him again. With all this that you have seen, even you do not know well how horrible he is, because, by Divine justice, he appears more horrible to the soul that is deprived of Me, and more or less according to the gravity of her sin.&quot;

St. Catherine lived in the mid-14th century. It's possible that earlier philosophers or theologians made similar statements, but I have been unable to find any.