sine qua non
noun
definition 1: (Latin) something necessary; indispensable item or condition.
Note:
This Latin expression means, literally, “without which, not.” In other words, without the “sine qua non,” the thing in question cannot exist.
Quotations:
“Conflict is the gadfly of thought. It stirs us to observation and memory. It instigates invention. It shocks us out of sheep-like passivity, and sets us at noting and contriving…conflict is a sine qua non of reflection and ingenuity.” John Dewey
“Chastity is the sine qua non of lucidity and concentration.” Dietrich Bonhoeffer