All the world’s a stage and most of us are desperately unrehearsed.

Sean O'Casey (1880-1964) Irish playwright [b. John Casey, a.k.a. Seán O'Cathaseaigh]

(Attributed)

Widely attributed to O'Casey, but I am unable to find a primary source or verifiable citation.

See Cicero (44 BC), Shakespeare (1599), Heywood (1612), Billings (1877), Brooks (1970), Watterson (1993).