To muse is to ponder or to think, and since the Muses are the source of inspiration for poetry, art, comedy, music, and dance in ancient Greek religion and myth, it might make sense to think of them also as the inspiration for deep thoughts. Except that they aren’t.
The muse that is the noun meaning “a source of inspiration” or, when capitalized, one of the nine Muses, indeed comes from the Greek name for them, which passed through Latin and French to English.
But the muse that is the verb meaning “to become absorbed in thought” comes from a different source: the Middle French word muse, meaning “the mouth of an animal” or “snout.” It’s assumed that the facial expression when one is thinking is what connects this word to absorption and reflection, and that the French verb had come to mean “to gape, to stare, to idle, to muse” because of the face one makes when lost in thought.
Though it may not share etymological roots with the Muses, the verb muse does have a relative in English that connects in a more literal way with their shared past: muzzle.
When we ponder, we think carefully about something. Another synonym is weigh, as in “to weigh a serious decision”—a word that connects with ponder more literally than you may think. Ponder came to English from a French word with the same meaning, ponderer, but its ultimate root is the Latin word pondus, meaning “weight.”
Other common words that derive from pondus have to do with things that are hanging, heavy, or a unit of weight itself:
Because we cannot see thoughts, the words we use to describe the process of thinking are usually figurative, like the difference in the uses of active in “running to keep active” and “an active imagination.” We often “turn over” an idea. Thoughts can nevertheless be (figuratively) agitating, which gets us to the root of cogitate. Cogitate means “to think carefully and seriously about something,” and it comes from the Latin cogitare (“to think”), itself formed from the combination of ¬co- meaning “together” and agitare meaning “to drive” or “to agitate”—the root of agitate in English and, in this case, another figurative use of language, since it could also mean “to turn over in the mind” in Latin.
Cogitate became the Latin-based verb synonym for the Old English-derived think, and cogitation the synonym for the noun thought. Here it’s used in the King James Bible:
Hitherto is the end of the matter. As for me Daniel, my cogitations much troubled me, and my countenance changed in me: but I kept the matter in my heart.
Other words derived from cogitare have fallen out of active use in English, but they show that this fancy way of saying “to think” was a rich source of vocabulary. These words were entered in Merriam-Webster’s Unabridged edition of 1934:
cogitabund “deep in thought; thoughtful”
cogitative “given to thought; meditative”
cogitativity “cognitive power or action”
Sometimes we “weigh" thoughts, sometimes we “turn them over," and other times they give us something to “chew on." At least that’s what the verb ruminate literally means: it comes from the Latin word ruminari, meaning “to chew the cud,” as in what cows do. Ruminari comes from the Latin word for the cow’s first stomach, rumen, and is also the root of the word for the category of mammals that have 3- or 4-chambered stomachs and two-toed feet, ruminants, which includes cattle, deer, giraffes, goats, and sheep.
Ruminate has been used as a fancy way to say “to think about” since the Renaissance in the 1500s, at a time when academic and philosophical writing was usually done by people with a strong background in Latin.
We distinguish between thoughts and ideas, and, unsurprisingly, there are verbs in English for producing both. The usage of these verbs, however, is extremely imbalanced: think is, of course, a fundamental part of our vocabulary and is very frequently used, but ideate is not.
You might think that ideate is simply some kind of annoying recent business jargon, but in fact its use in English dates back to the 1600s, when it referred to Platonic philosophy, meaning “to form an idea or conception of.” When referring to an abstract or perfect example of something, we also use a word related to idea, Platonic ideal.
Another related word is ideation, meaning “the capacity or the act of forming or entertaining ideas.” This word is used in specific contexts, such as in psychological assessments (“suicidal ideation”) and the creative aspect of technical jobs (“software-based ideation,” “digital strategy, ideation, and innovation.”) The fact is, ideate means something slightly different from think, since it expresses a clear goal: “to form an idea.” This is a useful distinction in fields like design and information technology:
“There’s a template for where all the numbers should be,” [Martin] Grann explains. You kind of feel it’s a little bit hard to ideate and to be creative when you have such strong guidelines and direction.”— Shaunecy Ferro, Co.Design, 9 October 2014
This is particularly true for the human-centered design process — empathize, define, ideate, prototype, test — as outlined by the Institute of Design at Stanford, also known as "the d.school"— Amanda Enayati, CNN.com, 19 June 2012
Smart is an Old English-derived word; intellectual is a Latin-derived word. Like most synonyms, they overlap rather than duplicate meanings. And like most pairs of words with one each from these particular family groups, the one with roots in Old English is the everyday, household word (“knowledgeable”) while the one with Latin roots is more fancy and hifalutin (“chiefly guided by the intellect rather than emotion”). There is a related and arguably fancier word meaning “thinking”: intellection. Intellection means “the act of the intellect” or “exercise of the intellect,” a synonym of thought and reasoning.
The greater emotional distance of many Latin-derived words in English makes intellection a perfect term for dispassionate analysis, and has been used in theological writing and literary criticism for centuries:
The severall opinions of philosophers concerning the manner how intellection is wrought or produced.— Thomas Jackson, A treatise containing the originall of vnbeliefe, misbeliefe, or misperswasions concerning the veritie, vnitie, and attributes of the Deitie, 1625
But time and again in her first two essay collections, Against Interpretation and Styles of Radical Will, she argued for a more sensuous, less intellectual approach to art. It was an irony lost on no one, except perhaps her, that she made those arguments in paragraphs that were marvels of strenuous intellection.— Richard Lacayo, TIME, 10 January 2005
Outside of these contexts, intellection serves a way of emphasizing thought or thinking in a positive way and contrasting it with the alternative:
Rather, [the fidget spinner] enables and even encourages the setting of one’s own interests above everyone else’s. It induces solipsism, selfishness, and outright rudeness. It does not, as the Rubik’s Cube does, reward higher-level intellection.— Rebecca Mead, The New Yorker, 12 May 2017
The Greek word meaning “to think” or “to perceive” came to English as noesis, meaning “purely intellectual knowledge” or “a process or act of thinking.” The adjective noetic means “of, relating to, or based on the intellect.” Its use in philosophical and psychological writing shows that it is perhaps the most abstract of our “thought” words:
As such, quantum theory has opened the door to a noetic, mind-based universe. Reality, we would infer, is mind-made.— Deepak Chopra, The Huffington Post, 29 October 2012
While-out-of-body experiences have the character of a perceptual illusion (albeit a complex and singular one), near-death experiences have all the hallmarks of mystical experience, as William James defines them passivity, ineffability, transience, and a noetic quality.— Oliver Sacks, Musicophilia, 2007
Noetic is also used in connection with the supernatural: the former astronaut Ed Mitchell founded a center for the study of paranormal phenomena and consciousness called the Institute of Noetic Sciences.
A more down-to-earth use of the word is as a synonym for “thoughtful” sometimes used for humor:
Someone recently asked if people actually understand my columns. I don't understand them sometimes. I attempt to be noetic, but can often come off as verbose and obtuse, if not borderline lugubrious. Until then, I'm doing my best to be compunctious.— Jim Magdefrau, Des Moines Register, 25 October 2017
Pensive comes from the French verb penser, meaning “to think.” The literal meaning of pensive, therefore, is “thoughtful,” but it came to English with a downcast attitude. Samuel Johnson defined the word this way in 1755:
Sorrowfully thoughtful; sorrowful; mournfully serious; melancholy
Shakespeare used pensive in this sense:
Now, brother of Clarence, how like you our choice, That you stand pensive, as half malcontent?— Henry VI, Part III, Act IV, Scene I
My leisure serves me, pensive daughter, now. My lord, we must entreat the time alone.— Romeo and Juliet, Act IV, Scene I
This melancholy mood continues today in our use of the word: though it can have the more neutral meaning of “musingly or dreamily thoughtful,” it also means “suggestive of sad thoughtfulness.”
The Latin word for brain was borrowed into English whole: cerebrum can refer either to the front part of the brain that is believed to be where thoughts occur or more generally as a synonym of brain itself. Scientists study both conscious and unconscious brain activity, and a technical term based on cerebrum for the latter, “unconscious cerebration,” was coined in the mid-19th century to distinguish it from what we might know of as “thinking.”
Cerebration (“mental activity,” “thought”) and the verb that derived from it a few years later, cerebrate (“to use the mind,” “to think”) have the technical, medical, and psychological overtones that come from Latin-derived vocabulary in a research field. Consequently, its use is sometimes distinctly technical:
Such exercise may well increase aerobic capacity, as these investigators have convincingly demonstrated, but does it stimulate cerebration or prevent boredom?— Samuel Vaisrub, JAMA Vol. 243 No. 20, 1980
And also used in a jocular way as a very formal-sounding synonym for “thought”:
Although the coining of a neologism is abundantly appealing, I cannot claim the word "feminal" as a product of my own cerebrations.— William Safire, I Stand Corrected, 1984
Its use can also convey a shade of emotional distance:
Nolan is now one of the greatest and most inventive movie technicians. He also lists the Argentinian writer Jorge Luis Borges near the top of the people who influenced him, which signals his taste for cerebration, and can be seen in some of his earlier movies, like Memento and Insomnia. But the most Borgesian quality in Nolan's work is his cool detachment from the world he describes. — Jonathan Raban, theStranger.com, 17 June 2017
One impediment to greater usage of cerebration is its similarity to celebration, which can make it easily misunderstood. In fact, much evidence shows clear misspellings: when you read about a “boisterous cerebration,” it should make you stop and think.
The Latin root word that gave us ratio and rational also gave us ratiocination, pronounced /rat-ee-oh-suh-NAY-shun/ or /rash-ee-oh-suh-NAY-shun/. It means “the process of exact thinking” or “a reasoned train of thought.” In Latin, ratio meant “reason” or “computation,” and the mathematical connotation of this word made it appealing for those describing a machinelike thinking process. Edgar Allan Poe used it to describe his story The Murders in the Rue Morgue, considered the first detective story as we now know them, as “a tale of ratiocination.” Unsurprisingly, the most famous character of the new genre was also the possessor of perhaps the most machinelike brain in fiction, Sherlock Holmes. And ratiocination is a favorite word used to describe him:
Holmes’s famous ratiocination is now at the service of a man of action.— David Denby, The New Yorker, 4 January 2010
One of the characters in the Ritchie film remarks that there is a fragility beneath all Holmes's logic and ratiocination, and it's true. Mr. Downey's character is as needy as he is superior.— Charles McGrath, The New York Times, 6 January 2010
Those cases -- and Sherlock Holmes's ratiocinations -- are fated to remain forever untold, mentioned in Dr. Watson's chronicles but never explained beyond these baroque references, with their nearly comic grotesqueries.— Edward Rothstein, The New York Times, 15 February 2014
The adjective ratiocinative is occasionally encountered in similar contexts:
Early in ''Sherlock Holmes'' — and also again, later on — the famous sleuth demonstrates his ratiocinative powers in a way undreamed of by his creator, Arthur Conan Doyle.— A.O. Scott, The New York Times, 25 December 2009
The verb ratiocinate is also sometimes seen:
But we're here to see Downey ratiocinate his way in and around the movie, and Ritchie indulges him and us.— Ty Burr, The Boston Globe, 25 December 2009
Ratiocinate is a pretty fancy way of saying “to think,” and usually draws attention to itself as a very technical and logical word. It received an unusual note at its definition in our Unabridged edition of 1934:
To reason discursively or according to a logical process ; —now usually humorous