When I receive a letter from a friend, I perceive its words, I become conscious of their meaning, I remember my relations to him; for instance, the time of our first meeting. But my thought proceeds. I wonder how he is getting along now, whether better or worse than myself, whether he has succeeded in overcoming through his greater energy the obstacles which retarded my progress. This is more than perception, imagination, or abstract consciousness. It is a _coherent process of thinking_. The best way of describing its characteristics is to tell what the opposite of _coherent_ thought is. First, coherent thought is not dreaming. The elements of a dream are of course united by something. But they are united only like the links of a chain. If the second link were removed, nothing would hold the first and the third together. This chain-like thought is frequent in the insane. The following is an example from Diefendorf’s _Psychiatry_:-- “My mother came for me in January. She had on a black bombazine of Aunt Jane’s. One shoestring of her own and got another from neighbor Jenkins. She lives in a little white house kitty corner of our’n. Come up with an old green umbrella ’cause it rained. You know it can rain in January when there is a thaw. Snow wasn’t more than half an inch deep, hog-killing time, they butchered eight that winter, made their own sausages, cured hams, and tried out their lard. They had a smoke house. [Question: But how about your leaving Hartford?] She got up to Hartford on the half-past eleven train and it was raining like all get out. Dr. Butler was having dinner, codfish, twasn’t Friday, he ain’t no Catholic, just sat with his back to the door and talked and laughed and talked.” 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. This recurring kind of thought, too, is a frequent symptom in cases of mental derangement; for example, as a continuously present desire to kill somebody, or as the permanent idea of one’s own sinfulness and worthlessness.