I do want to acknowledge that sanity is a thing in cosmic horror; however, as I have fairly recently read Call of Cthulhu (a year+ ago) and At the Mountains of Madness (yesterday) I want to point out that narrator / protagonist in both stories does not suffer any madness (other characters do). And that is the role I see the PCs, the protagonist of the story. They investigate and fight (futilely) the horrors, but are rarely driven insane by them.
I think this isn't obvious.I just reread At the Mountains of Madness yesterday and Call of Cthulhu a year+ or so ago. The narrator/protagonist in those stories is disturbed by the events and what they uncover, but they are not noticeably worse off than from where they began.
Here is the second paragraph of The Call of Cthulhu:
Theosophists have guessed at the awesome grandeur of the cosmic cycle wherein our world and human race form transient incidents. They have hinted at strange survivals in terms which would freeze the blood if not masked by a bland optimism. But it is not from them that there came the single glimpse of forbidden eons which chills me when I think of it and maddens me when I dream of it. That glimpse, like all dread glimpses of truth, flashed out from an accidental piecing together of separated things—in this case an old newspaper item and the notes of a dead professor. I hope that no one else will accomplish this piecing out; certainly, if I live, I shall never knowingly supply a link in so hideous a chain. I think that the professor, too intended to keep silent regarding the part he knew, and that he would have destroyed his notes had not sudden death seized him.
And here is the second-last paragraph:
That was the document I read, and now I have placed it in the tin box beside the bas-relief and the papers of Professor Angell. With it shall go this record of mine—this test of my own sanity, wherein is pieced together that which I hope may never be pieced together again. I have looked upon all that the universe has to hold of horror, and even the skies of spring and the flowers of summer must ever afterward be poison to me. But I do not think my life will be long. As my uncle went, as poor Johansen went, so I shall go. I know too much, and the cult still lives.
This is someone who is noticeably worse off, in my view.
These paragraphs are from the closing pages of At the Mountains of Madness:
If the sculptured maps and pictures in that prehuman city had told truly, these cryptic violet mountains could not be much less than three hundred miles away; yet none the less sharply did their dim elfin essence appear above that remote and snowy rim, like the serrated edge of a monstrous alien planet about to rise into unaccustomed heavens. Their height, then, must have been tremendous beyond all comparison—carrying them up into tenuous atmospheric strata peopled only by such gaseous wraiths as rash flyers have barely lived to whisper of after unexplainable falls. Looking at them, I thought nervously of certain sculptured hints of what the great bygone river had washed down into the city from their accursed slopes—and wondered how much sense and how much folly had lain in the fears of those Old Ones who carved them so reticently. I recalled how their northerly end must come near the coast at Queen Mary Land, where even at that moment Sir Douglas Mawson's expedition was doubtless working less than a thousand miles away; and hoped that no evil fate would give Sir Douglas and his men a glimpse of what might lie beyond the protecting coastal range. Such thoughts formed a measure of my overwrought condition at the time—and Danforth seemed to be even worse. . . .
Danforth, released from his piloting and keyed up to a dangerous nervous pitch, could not keep quiet. I felt him turning and wriggling about as he looked back at the terrible receding city, ahead at the cave-riddled, cube-barnacled peaks, sidewise at the bleak sea of snowy, rampart-strewn foothills, and upward at the seething, grotesquely clouded sky. It was then, just as I was trying to steer safely through the pass, that his mad shrieking brought us so close to disaster by shattering my tight hold on myself and causing me to fumble helplessly with the controls for a moment. A second afterward my resolution triumphed and we made the crossing safely—yet I am afraid that Danforth will never be the same again.
I have said that Danforth refused to tell me what final horror made him scream out so insanely—a horror which, I feel sadly sure, is mainly responsible for his present breakdown. We had snatches of shouted conversation above the wind's piping and the engine's buzzing as we reached the safe side of the range and swooped slowly down toward the camp, but that had mostly to do with the pledges of secrecy we had made as we prepared to leave the nightmare city. Certain things, we had agreed, were not for people to know and discuss lightly—and I would not speak of them now but for the need of heading off that Starkweather-Moore Expedition, and others, at any cost. It is absolutely necessary, for the peace and safety of mankind, that some of earth's dark, dead corners and unplumbed depths be let alone; lest sleeping abnormalities wake to resurgent life, and blasphemously surviving nightmares squirm and splash out of their black lairs to newer and wider conquests.
Danforth, released from his piloting and keyed up to a dangerous nervous pitch, could not keep quiet. I felt him turning and wriggling about as he looked back at the terrible receding city, ahead at the cave-riddled, cube-barnacled peaks, sidewise at the bleak sea of snowy, rampart-strewn foothills, and upward at the seething, grotesquely clouded sky. It was then, just as I was trying to steer safely through the pass, that his mad shrieking brought us so close to disaster by shattering my tight hold on myself and causing me to fumble helplessly with the controls for a moment. A second afterward my resolution triumphed and we made the crossing safely—yet I am afraid that Danforth will never be the same again.
I have said that Danforth refused to tell me what final horror made him scream out so insanely—a horror which, I feel sadly sure, is mainly responsible for his present breakdown. We had snatches of shouted conversation above the wind's piping and the engine's buzzing as we reached the safe side of the range and swooped slowly down toward the camp, but that had mostly to do with the pledges of secrecy we had made as we prepared to leave the nightmare city. Certain things, we had agreed, were not for people to know and discuss lightly—and I would not speak of them now but for the need of heading off that Starkweather-Moore Expedition, and others, at any cost. It is absolutely necessary, for the peace and safety of mankind, that some of earth's dark, dead corners and unplumbed depths be let alone; lest sleeping abnormalities wake to resurgent life, and blasphemously surviving nightmares squirm and splash out of their black lairs to newer and wider conquests.
While it is Danforth and not the narrator who goes mad, the narrator is clearly disturbed. And seems to think that he is worse off for his knowledge, which he shares only to head off the new expedition.
There is a consistent theme of being "damaged" by knowledge that reveals some horrible truth about human history and culture and its relationship to the universe as it really is.