Why do we say "Man does not live by bread alone"?

Meaning:

People have spiritual needs as well as physical needs.

Background:

"And he humbled thee, and suffered thee to hunger, and fed thee with manna, which thou knewest not, neither did thy fathers know; that he might make thee know that man doth not live by bread only, but by every word that proceedeth out of the mouth of the Lord doth man live." - Deuteronomy 8:3, King James Bible.

"Jesus answered, 'It is written: Man does not live on bread alone, but on every word that comes from the mouth of God." - Matthew 4:4, New International Bible.

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