How can you see that Ralph has lost his clarity of thought?
In "Beast From Water", Ralph for the first time tries to logically think about what to say at the next assembly, and how to hold the group together. He finds it difficult:
He lost himself in a maze of thoughts that were rendered vague by his lack of words to express them...
Only, decided Ralph as he faced the chief’s seat, I can’t think. Not like Piggy.
Later in the novel, Ralph is again trying to think, and this time finds it incredibly difficult to get things clear - Golding's simile is striking, and quite famous:
A strange thing happened in his head. Something flittered there in front of his mind like a bat’s wing, obscuring his idea.
Literally obscured, Ralph can hardly see in front of him, something stops the clarity of the thought. The thought, earlier expressed, is "Smoke" - Ralph's starting to forget about the signal fire. By the end of the novel, alone in the jungle, Ralph's thoughts are a mismash of half-remembered thoughts, quotes from Simon, and thoughts addressed to himself. He's a mess:
The seconds lengthened. Ralph was looking straight into the savage’s eyes.
You’ll get back.
Now he’s seen you. He’s making sure. A stick sharpened.
Hope it helps!