Sine qua non or conditio sine qua non (plural sine quibus non) was originally a Latin legal term for "(a condition) without which it could not be" or "but for..." or "without which (there is) nothing." It refers to an indispensable and essential action, condition, or ingredient.

As a Latin term, it occurs in the work of Boethius, and originated in Aristotelian expressions.^[1] In recent times it has passed from a merely legal usage to a more general usage in many languages, including English, German, French, Italian, Spanish, etc. In Classical Latin the correct form uses the word condicio, but nowadays the phrase is sometimes found to be used with conditio, which has a different meaning in Latin ("foundation"). The phrase is also used in economics, philosophy and medicine.

An example of the term's usage was annotated in H.W. Brand's biography of Andrew Jackson. The book included a toast given by Jackson on the occasion of his receiving an honorary doctorate from Harvard University. The President responded to his listeners, "E pluribus unum, my friends. Sine qua non. "A recent example come from Javier Solana who said that the arrest of Radovan Karad?i? was a sine qua non condition for Serbia joining the European Union and "it has been a very important step to move closer to Europe."

It also appears in the commentary on Article 59 of the Fourth Geneva Convention on the protection of civilians during a time of war. In this case the sine qua non refers to the assurance that relief aid will go to the civilian population and not be diverted towards "the benefit of the Occupying Power."^[2].

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