Why do we say "Hope for the best and prepare (or plan) for the worst"?

Meaning:

Be optimistic but also be prepared for all possibilities.

Background:

Around 46 BC, Cicero wrote to a friend saying, "you must hope for the best"; but the first known use of the full expression is in The Tragedie of Gorbuduc by Thomas Norton and Thomas Sackville (1561) which was performed by the Gentlemen of the Inner Temple before Queen Elizabeth I in 1562.

It is believed that Norton wrote the first three acts and Sackville the last two. It the earliest known tragedy written in English and the first full length play composed in blank verse. Not considered a work of great literary merit, the play's subject matter is an ancient British monarch who quarrels with his two sons. It warns the young Queen of the dangers of misgovernment and depicts a family and a country torn apart by civil war.

A bit over forty years later, Shakespeare wrote King Lear (first produced in 1605). At first glance it might seem that he took the idea of a king quarreling with his children from The Tragedie of Gorbuduc and upped the ante from two sons to three daughters. But actually, his tragedy is based on the story of Leir, a legendary king of the Britons who makes his first appearance in Geoffrey of Monmouth's Historia Regum Britanniae written in the 12th century. According to Geoffrey, Leir and his three daughters, Goneril, Regan and Cordelia, were descendants of Brutus of Troy (the legendary founder of Britain), who in turn was a descendant of the Trojan hero Aeneas.

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