At the beginning of the short story "The Resident Patient", Arthur Conan Doyle narrates the following exchange between Dr. Watson and Mr. Holmes. Dr. Watson is absorbed in contemplation when, Holmes interrupts him with:
"You are right, Watson," said he. "It does seem a very preposterous way of settling a dispute."
"Most preposterous!" I exclaimed, and then, suddenly realizing how he had echoed the inmost thought of my soul, I sat up in my chair and stared at him in blank amazement.Dr. Watson is startled by Holmes' telepathy. Holmes goes on to explain:
"Then I will tell you. After throwing down your paper, which was the action which drew my attention to you, you sat for half a minute with a vacant expression. Then your eyes fixed themselves upon your newly-framed picture of General Gordon, and I saw by the alteration in your face that a train of thought had been started. But it did not lead very far. Your eyes turned across to the unframed portrait of Henry Ward Beecher which stands upon the top of your books. You then glanced up at the wall, and of course your meaning was obvious. You were thinking that if the portrait were framed it would just cover that bare space and correspond with Gordon's picture over there."
"You have followed me wonderfully!" I exclaimed.
"So far I could hardly have gone astray. But now your thoughts went back to Beecher, and you looked hard across as if you were studying the character in his features. Then your eyes ceased to pucker, but you continued to look across, and your face was thoughtful. You were recalling the incidents of Beecher's career. I was well aware that you could not do this without thinking of the mission which he undertook on behalf of the North at the time of the Civil War, for I remember you expressing your passionate indignation at the way in which he was received by the more turbulent of our people. You felt so strongly about it that I knew you could not think of Beecher without thinking of that also. When a moment later I saw your eyes wander away from the picture, I suspected that your mind had now turned to the Civil War, and when I observed that your lips set, your eyes sparkled, and your hands clinched, I was positive that you were indeed thinking of the gallantry which was shown by both sides in that desperate struggle. But then, again, your face grew sadder; you shook your head. You were dwelling upon the sadness and horror and useless waste of life. Your hand stole towards your own old wound, and a smile quivered on your lips, which showed me that the ridiculous side of this method of settling international questions had forced itself upon your mind. At this point I agreed with you that it was preposterous, and was glad to find that all my deductions had been correct."While we may lack Holmes' deductive prowess, we are all very familiar with such trains of thought.
You listen to a song on the radio. It reminds you of the last time you heard it, and who you were with then. That brings other memories of your companions, and within a few simple steps, you've embarked on a bumpy trajectory of thoughts that leads you far away from the original stimulus.
It doesn't have to be solitary contemplation. The same phenomenon occurs in conversations between friends, for example. You are talking about something. One thing leads to another, and very soon the original history of the train of thoughts gets obscured, unless one takes particular effort to pause, and reconstruct it.
The act of reconstruction can sometimes be insightful in weird way.
On a recent episode of "A Way with Words", Grant was talking about his baldness, and Martha brought up the word "callow", which originally meant "bald".
The term callow goes back to Old English calu, meaning “bald.” The original sense of callow referred to young birds lacking feathers on their heads, then referred to a young man’s down cheek, and eventually came to mean “youthful” or “immature.”
The evolution from "bald" to "immature" does not seem direct, unless one appreciates the trajectory of the word.