From Galaxies Like Grains of Sand (1960) by Brian Aldiss:
For a long minute, Jandanagger was silent, searching for the key phrases of explanation.
“You have learned as much as you have very rapidly,” he said. “By not-understanding and then by well-understanding, you have made yourself one of the true citizens of the Galaxy. But you have only taken leap X; now you must take leap X¹º. Prepare yourself.”
“I am prepared.”
“All that you have learned is true. Yet there is a far greater truth, a truer truth. Nothing exists in the ultimate sense; all is illusion, a two-dimensional shadow play on the mist of space-time. Yinnisfar itself means ‘illusion.’”
“But the clawed thing…”
“The clawed thing is why we fare even farther ahead into the illusion of space. It is real. Only the Galaxy as you previously misinterpreted it is unreal, being but a configuration of mental forces. That monster, that thing you sensed, is the residue of slime of the evolutionary past still lingering – not outside you, but in your mind. It is from that we must escape. We must grow from it.”
More explanation followed, but it was beyond Farro. In a flash, he saw that Jandanagger, with an eagerness to experiment, had driven him too far and too fast. He could not make the last leap; he was falling back, toppling into non-being. Somewhere within him, the pop-thud-pop sound of bursting arteries began. Others would succeed where he had failed, but, meanwhile, the angry claws were reaching from the heavens for him – to sunder, not to rescue.