A game that he had thought existed only in the minds of the two men!

At one point, the music exists in the composer's mind.

Whatever this poor creature saw existed only in her own mind.

Maybe all that, like the creature, existed only in his mind.

"Likewise, the future exists only in your mind because it has not happened."

Or are the lies evidence that the ghosts exist only in her mind?

But the night was still so silent that she knew at once the scream she'd heard existed only in her mind.

That is to say: as those passengers' thoughts exist in my mind.

A belief is an internal thought or memory which exists in one's mind.

The nation was never fully established, and only existed in the minds of a few.