From Traveling in Mark Twain (Richard Bridgman, 1987):

Having long pondered, without resolution, the meaning of what he had observed in his travels through the world, Twain told his old friend and clergyman Joe Twichell, in a letter of July 28, 1904, that at least “a part of each day—or night” the world vanished absolutely for him, leaving him no more than a thought drifting aimlessly in emptiness. At such moments it seemed to him that everything was “NON-EXISTENT. That is, that there is nothing . That there is no God and no universe; that there is only empty space, and in it a lost and homeless and wandering and companionless and indestructible Thought . And I am that thought.” (MSM , 30).