In memory of.

Since the days of the Roman empire, the words *In memoriam*, followed by a name, have been found on monuments and gravestones. They may also appear in the dedication of a book or poem; Alfred Tennyson's greatest poem is his immense *In Memoriam,* written over a period of 17 years to mourn the death of his dear friend Arthur Hallam.

A great work, especially the greatest achievement of an artist, composer, or writer.

The greatest work of a great artist may be hard to agree on. Many would pick Rembrandt's *The Night Watch,* Mozart's *Don Giovanni,* Ovid's *Metamorphoses,* Dante's *Divine Comedy,* Wren's St. Paul's Cathedral, and Michelangelo's Sistine Chapel murals. But for Shakespeare, would it be *Hamlet* or *King Lear*? For Mahler, *The Song of the Earth* or the Ninth Symphony? For the Marx Brothers, *A Day at the Races* or *A Night at the Opera*?

A reminder of mortality, especially a human skull symbolizing death.

*Memento mori* literally means “Remember you must die.” The early Puritan settlers were particularly aware of death and fearful of what it might mean, so a Puritan tombstone will often display a memento mori intended for the living. These death's-heads or skulls may strike us as ghoulish, but they helped keep the living on the straight and narrow for fear of eternal punishment. In earlier centuries, an educated European might place an actual skull on his desk to keep the idea of death always present in his mind.

The literal meaning of *habeas corpus* is “You shall have the body”—that is, the judge must have the person charged with a crime brought into the courtroom to hear what he's been charged with. Through much of human history, and in many countries still today, a person may be imprisoned on the orders of someone in the government and kept behind bars for years without ever getting a chance to defend himself, or even knowing what he's done wrong. In England, the right to be brought before a judge to hear the charges and answer them was written into law over 300 years ago, and the U.S. adopted the British practice in its Constitution.

The temporary rigidity of muscles that sets in after death.

*Rigor mortis,* which translates from Latin as “stiffness of death,” sets in quickly and usually ends three or four days after death. The condition results from a lack of certain chemicals in the muscles; it may be affected by muscular activity before death as well as the external temperature. Mystery writers frequently make use of rigor mortis as a means by which the detective or the examiner can determine the time of the victim's death, which often turns out to be all-important in solving the case.

An essential thing.

*Sine qua non* can be translated literally as “Without which, not.” Though this may sound like gibberish, it means more or less “Without (something), (something else) won't be possible.” *Sine qua non* sounds slightly literary, and it shouldn't be used just anywhere. But it actually shows up in many contexts, including business (“A solid customer base is the sine qua non to success”), show business (“A good agent is a sine qua non for an actor's career”), and politics (“His support was really the sine qua non for her candidacy”).

(1) The mind in its blank or unmarked state before receiving any impressions from outside. (2) Something existing in its original pure state.

In ancient Rome, a student in class would write on a wax-covered wooden tablet, or *tabula*, using a sticklike implement. At the end of the day, the marks could be scraped off, leaving a fresh, unmarked tablet—a *tabula rasa* —for the next day's lessons. But even before the Romans, the Greek philosopher Aristotle had called the mind at birth an “unmarked tablet.” We still use the term today, but usually not very seriously; with what we know about biology and genetics, most of us don't really think there's nothing in a mind at birth.

An unexplored country or field of knowledge.

When Roman mapmakers drew a land area that no one had yet explored, they often labeled it “Terra Incognita”—that is, “Unknown Territory”—and the term continued to be used for centuries afterward. When Columbus and his successors first crossed the Atlantic, they entered upon terra incognita, a land that came to be called the “New World.” But the term is just as useful for mental exploration. For most of us, subjects such as particle physics, French 17th-century drama, and soil mechanics are terra incognita, and we can only hope to live long enough to be able to explore some of them someday. ```