KENT TANKERSLEY, of Helsinki, Finland, writes, "I work in a European office of a multinational company. After I recently wrote the somewhat clichéd phrase 'We cannot understate the importance of ... ,' some of my colleagues, who are not native English-speakers, pointed out that this didn't make sense. Although the phrase sounded perfectly okay to me and the only other native English-speaker in the office, I had to admit that, logically, the literal meaning was the opposite of what I intended. Obviously, it should be 'We cannot overstate the importance of ...' Puzzled by why the phrase had sounded so natural in the first place, I did a Google search for cannot understate the importance and found that it is indeed used in reputable documents.
"Doing the same for cannot overstate the importance, I found that, unsurprisingly, this correct usage is seen more frequently. Is it wrong to use cannot understate the importance? Or is it so widely accepted to mean the opposite of its literal meaning that it doesn't matter, because everyone understands it anyway?"
Cannot understate and cannot overstate are like architectural elements in an M. C. Escher drawing: if you like, you can flip-flop them in your mind. The trick is done by cannot, which has two meanings. Think of Parson Weems's tale in which the young George Washington declared, "I can't tell a lie." Of course Washington was physically capable of uttering a false statement; by can't, he meant he chose not to. Can't, or cannot, can mean something very much like must not—and if it means that, cannot understate the importance of makes sense. Clear communication is subverted, though, when antithetical statements mean the same thing. Cannot overstate is more commonly seen and heard, as you say—in fact, it's much more common. Why not use it from now on?
AMI DIGIACINTO, of Burbank, California, writes, "I know that the symbol for medicine, a winged staff with two snakes twined around it, is called a caduceus. I've noticed that the symbol is slightly different for 'lower-level' medical professions, such as paramedic or nurse. On the symbol for the latter there is only one snake. Is the duc in caduceus the root of the Latin word for 'two' (as in 'two snakes'), and does the latter symbol therefore have a different name?"
In fact the one-snake staff is the symbol that's more directly relevant to medicine, because it originally belonged to Asklepios (his Greek name), or Aesculapius (as the Romans called him)—the god or demigod or at any rate a practitioner of medicine and healing. The snake, because it sheds its skin, was a symbol of rejuvenation, and Aesculapius was thought to appear at times in the form of a snake. When there's only one snake, it's the staff of Aesculapius. The American Medical Association and the National Registry of Emergency Medical Technicians, among other organizations, use this as their symbol.
As for caduceus, in Latin the word means a herald's wand or staff; the duc is just a syllable and does not signify "two." Hermes (Greek), or Mercury (Roman), carried a caduceus. He was not only the patron of commerce, travel, and thievery but also the messenger of the other gods. Various stories are told about why Mercury's caduceus has snakes twined around it—but none of them has much to do with medicine.
In 1902, even as the coat of arms of the U.S. Army Medical Corps continued to bear the staff of Aesculapius, the corps added the caduceus to its officers' uniforms—not to symbolize the medical profession but to indicate neutrality in combat. According to an article by Lieutenant Colonel F. H. Garrison, M.C., U.S. Army, that appeared in the Bulletin of the Medical Library Association in 1919, "On the firing line, the medical officer, if the Geneva Convention is observed, is protected by his non-combatant status, just as the caduceus-bearing herald of ancient Rome was immune from attack." Today the caduceus is the official U.S. Army medical insignia. However, the British Royal Medical Corps uses the staff of Aesculapius.
MARY WALDMAN, of Bellevue, Washington, writes, "Why are the local business organizations across the country called 'chambers of commerce'? 'Rooms' of commerce? There must be a logical reason for the use of chamber, but what it is I can't imagine."
You're right that chamber did first mean "room," but as long ago as the fourteenth century the word began to be used to refer to groups that met in particular rooms. When you think about it, the use of this term makes no more and no less sense than referring to the houses of a legislature—or calling the President and his advisers the White House.
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