"If there are two or more ways to do something, and one of those ways can result in a catastrophe, then someone will do it that way."
Origin of Murphy's LawBorn in 1917, Edward A. Murphy, Jr. was one of the engineers on the rocket-sled experiments that were done by the United States Air Force in 1949 to test human acceleration tolerances (USAF project MX981). One experiment involved a set of 16 accelerometers mounted to different parts of the subject's body. There were two ways each sensor could be glued to its mount. Of course, somebody managed to install all 16 the wrong way around. Murphy then made the original form of his pronouncement, which the test subject (Major John Paul Stapp) quoted at a news conference a few days later.
Within months, "Murphy's Law" had spread to various technical cultures connected to aerospace engineering, and finally reached the Webster's dictionary in 1958.
The traditional version of Murphy's Law ("anything that can go wrong, it will") is actually "Finagle's Law of Dynamic Negatives." Finagle's Law was popularized by science fiction author Larry Niven in several stories depicting a frontier culture of asteroid miners; this "Belter" culture professed a religion and/or running joke involving the worship of the dread god Finagle and his mad prophet Murphy.
VariationsMurphy's law has taken on many different formulations. In 1952, the proverb was phrased "Anything That Can Possibly Go Wrong, Does" in the epigraph of John Sack's The Butcher: The Ascent of Yerupaja. Possibly the earliest printed use of Murphy's name in connection with the law is in Lloyd Mallan's 1955 book, Men, Rockets and Space Rats: "Colonel Stapp's favorite takeoff on sober scientific laws Murphy's Law, Stapp calls it 'Everything that can possibly go wrong will go wrong'".
Yet another formulation can be found in the Jargon File, which states that the "correct, original" law is "If there are two or more ways to do something, and one of those ways can result in a catastrophe, then someone will do it."
Regardless of the exact composition and origin of the phrase, its spirit embodies the principle of defensive design anticipating the mistakes the end-user is likely to make. Murphy's g-force sensors failed because there existed two different ways to connect them; one way would result in correct readings, while the other would result in no readings at all. The end-user Murphy's assistant, in the historical account had a choice to make when connecting the wires. When the wrong choice was made, the sensors did not do their job properly.
In most well-designed technology intended for use by the average consumer, incorrect connections are made difficult. For example, the 3.5-inch floppy disk used in many personal computers will not easily fit into the drive unless it is oriented correctly. In contrast, the older 5.25-inch floppy disk could be inserted in a variety of orientations that might damage the disk or drive. The newer CD-ROM technology permits one incorrect orientation the disc may be inserted upside-down. A defensive designer knows that if it is possible for the disc to be inserted the wrong way, someone will eventually try it.
From its initial public announcement, Murphy's law quickly spread to various technical cultures connected to aerospace engineering. Before long, variants had passed into the popular imagination, changing as they went. Generally, the spirit of Murphy's law captures the common tendency to emphasize the negative things that occur in everyday life; in this sense, the law is typically formulated as some variant of "If anything can go wrong, it will", a variant often known as Finagle's law or Sod's law (chiefly British). An often-quoted example of this tendency to emphasize negatives is that whenever a buttered slice of bread falls on the floor, people tend to remember more vividly the times that it fell buttered-side-down, since a buttered-side-up landing is of lesser consequence. Hence, one gets the impression that the bread always falls buttered-side-down, regardless of the actual probability of either happening. Laws such as Murphy's are a direct expression of such seeming perversities in the order of the universe.
Additional mutations of the law and its corollaries have developed, many of them meta-laws in some way, either through some form of self-reference or referral to other laws or analogies. For instance, the buttered-bread analogy could be further extended: "The chance of a dropped slice of bread landing buttered-side down on a new carpet is proportional to the price of the carpet". (If the buttered side falls facing up, then obviously the wrong side is buttered.) A further example is Murphy's Ultimate Corollary: "If it could have gone wrong earlier and it did not, it ultimately would have been beneficial for it to have". John Gall's systemantics offers further expansion of Murphy's law.
Some state that Murphy's law cannot operate as a subset of something useful; for example: "It will start raining as soon as I start washing my car, except when I wash the car for the purpose of causing rain". O'Toole's commentary on Murphy's law is: "Murphy was an optimist!" These mutant versions demonstrate Murphy's law acting on itself, or perhaps Finagle's law acting on Murphy's law.