What Hath me? by Henry Kuttner

For a long time he had only his thoughts for company. They were not pleasant. He felt alone, as he had never felt so utterly lonely and deserted before anywhere in the System. Now he realized that even since his landing on Asgard, he had had companionship of a sort -- that the twin voices murmuring in his brain had been more real than he had realized. A living warmth, a sense of -- of presence -- had been with him then.

But it was gone now. Its absence left a black void within him. He stood alone.