Camel's nose

The camel's nose is a metaphor for a situation where permitting some small undesirable situation will allow gradual and unavoidable worsening. A typical usage is this, from U.S. Senator Barry Goldwater in 1958:

This bill and the foregoing remarks of the majority remind me of an old Arabian proverb: "If the camel once gets his nose in the tent, his body will soon follow." If adopted, the legislation will mark the inception of aid, supervision, and ultimately control of education in this country by the federal authorities.

According to Geoffrey Nunberg, the image entered the English language in the middle of the 19th century. An early example is a fable printed in 1858 in which an Arab miller allows a camel to stick its nose into his bedroom, then other parts of its body, until the camel is entirely inside and refuses to leave. Lydia Sigourney wrote another version, a widely reprinted poem for children, in which the camel enters a shop because the workman does not forbid it at any stage.

The 1858 example above says, "The Arabs repeat a fable," and Sigourney says in a footnote, "To illustrate the danger of the first approach of evil habit, the Arabs have a proverb, "Beware of the camel's nose". However, Nunberg could not find an Arab source for the saying and suspected it was a Victorian invention.

An early citation with a tent is "The camel in the Arabian tale begged and received permission to insert his nose into the desert tent." By 1878, the expression was familiar enough that part of the story could be left unstated. "It is the humble petition of the camel, who only asks that he may put his nose into the traveler's tent. It is so pitiful, so modest, that we must needs relent and grant it."

In a 1915 book of fables by Horace Scudder, the story, titled The Arab and His Camel, ends with the moral: "It is a wise rule to resist the beginnings of evil."

There are a number of other metaphors and expressions which refer to small changes leading to chains of events with undesirable or unexpected consequences, differing in nuances.

• Foot in the door - a persuasion technique
• Slippery slope - an argument, sometimes fallacious
• "The thin end of the wedge"'
• Domino effect
• For Want of a Nail (proverb) - the claim that large consequences may follow from inattention to small details
• Boiling frog
• "Give them an inch; they'll take a mile" The original saying goes "Give them an inch, and they'll take an ell".
• In Chinese culture, the "inch-mile" saying corresponds to the expression 得陇望蜀 (De Long Wang Shu), which is a quotation from the Book of Later Han about a Chinese general who took over Long (now Gansu) only to pursue further southwards into Shu (now Sichuan).

For comparison, positive consequences may start from small acts, and there is a similar set of sayings like Lao Tzu's Tao Te Ching: "A journey of a thousand miles begins with a single step" (or "A journey of a thousand li begins with a single step").

Relating this sentiment in idiom to scientific observation, the notion that large-scale phenomena may be affected by tiny initial incidents is the essence of chaos theory. However, in all the examples above, the result of the tiny initial incident is supposed to be predictable, unlike in chaos theory.