Hermann Ebbinghaus,
Psychology: an elementary text-book
“ In other cases, mere similarity of words of different meaning, rhyme, familiar questions, or spatial contiguity of things lead consciousness from one idea to a second, from the second to the third, and so on, without any common tie which would unite all these ideas into one system.Coherent thought, secondly, is no endless recurring of the same few ideas, as when I am brooding over something, when a song which I have heard occupies my mind and gives me no peace, when the thought of having possibly failed to lock the door properly prevents me from sleeping. ”

