The man on the Clapham omnibus
The man on the Clapham omnibus is a hypothetical reasonable person, used by the courts in English law where it is necessary to decide whether a party has acted as a reasonable person would — for example, in a civil action for negligence. The man on the Clapham omnibus is a reasonably educated and intelligent but nondescript person, against whom the defendant's conduct can be measured. The concept was used by Greer LJ in the case of Hall v. Brooklands Auto-Racing Club (1933) to define the standard of care a defendant must live up to in order to avoid being found negligent.
The phrase was first put to legal use in a reported judgment by Sir Richard Henn Collins MR in the 1903 English Court of Appeal libel case, McQuire v. Western Morning News. He attributed it to Lord Bowen, said to have coined it as junior counsel defending the Tichborne Claimant case in 1871. Brewer's also lists this as a possible first use.
It may be derived from the phrase "Public opinion ... is the opinion of the bald-headed man at the back of the omnibus," a description by the 19th-century journalist Walter Bagehot of a normal London man. Clapham, in South London, was at the time a nondescript commuter suburb seen to represent "ordinary" London. Omnibus is now rather an archaic term for a public bus, but was in common use by the judiciary at the beginning of the 20th century.
-  2 KB 100 (CA) at 109 per Collins MR
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