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The phrase “No man is an island” comes from:

John Donne, Devotions upon Emergent Occasions, Meditation XVII (1624)

Full citation (modern format):

Donne, John. Devotions upon Emergent Occasions and Several Steps in My Sickness. Meditation XVII, 1624.

Famous passage:

“No man is an island, entire of itself; every man is a piece of the continent, a part of the main…”

This meditation is also the source of the phrase “for whom the bell tolls,” later used by Ernest Hemingway.