The simplest explanation of something is apt to be the correct one. This principle is named for the English scholar William of Occam (or Ockham), who lived from 1280 to 1349. A Franciscan monk, he so angered Pope John XXII through both his writings on the nature of knowledge and his defense of his order’s vow of poverty that he was excommunicated. William, whom his colleagues called Doctor Singularis et Invincibilis (“singular and invincible doctor”), put his principle in Latin: Entia non sunt multiplicanda praeter necessitatem, “Entities should not be unnecessarily multiplied.” In effect, he held that any unnecessary parts of a subject being analyzed should be eliminated. Obviously, this could simply be called Occam’s Principle, and indeed, the razor did not enter into it until a French philosopher, Etienne Bonnot de Condillac, in 1746 called it Rasoir des Nominaux, “the razor of the nominalists,” that is, cutting through complicated arguments to reach the truth. In 1836 Sir William Hamilton, lecturing on metaphysics and logic, put the two ideas together, saying, “We are therefore entitled to apply Occam’s razor to this theory of causality.” While some may believe that this phrase, with its ancient and rather abstruse origin, is obsolete, novelist Archer Mayor clearly disagreed, for he entitled his 1999 murder mystery Occam’s Razor.

In a critical or dangerous predicament. This analogy dates from Homer’s time (Iliad, ca. 850 b.c.): “To all it stands on a razor’s edge, either woeful ruin or life for the Achaeans.” W. Somerset Maugham used it as the title of a philosophical novel (The Razor’s Edge, 1944) exploring the meaning of life. Alan White used it in The Long Silence (1976): “He was living on a razor’s edge. Sooner or later, the Germans were going to begin to suspect.”

Singularly keen or cutting; also, mentally acute. This simile has largely supplanted the earlier sharp as a razor, needle, vinegar, and thorn, the last dating from the fifteenth century and appearing in John Ray’s 1670 proverb collection. The current cliché dates from the late nineteenth or early twentieth century and appeared in a 1912 issue of Dialect Notes: “They won’t fool him; he’s sharp as tacks.”