Segal's law is an adage that states: A man with a watch knows what time it is.
[1]At surface level, the adage emphasizes the consistency that arises when information comes from a single source and points out the potential pitfalls of having too much conflicting information.
However, the underlying message is to question the apparent certainty of anyone who only has one source of information.
The man with one watch has no way to identify error or uncertainty.
The saying was coined by the San Diego Union on September 20, 1930: "Confusion.—Retail jewelers assert that every man should carry two watches.