No one is self-sufficient; everyone relies on others. This saying comes from a sermon by the seventeenth-century English author John Donne.
no man is an island
Human beings necessarily depend on one another, as in You can't manage this all by yourself; no man is an island. This expression is a quotation from John Donne's Devotions (1624): "No man is an Island, entire of it self; every man is a piece of the Continent, a part of the main."