Condicio sine qua non

Sine qua non (/ˌsaɪni kweɪ ˈnɒn/; Latin: [ˈsine kwaː ˈnoːn]) or conditio sine qua non (plural: conditiones sine quibus non) is an indispensable and essential action, condition, or ingredient. It was originally a Latin legal term for "[a condition] without which it could not be", or "but for..." or "without which [there is] nothing". "Sine qua non" causation is the formal terminology for "but-for" causation.

As a Latin term, it occurs in the work of Boethius, and originated in Aristotelian expressions. In Classical Latin, the form uses the word condicio (from the verb condico, condicere, to agree upon), but in later Latin the phrase is also used with conditio (condition).

It has passed from a merely legal usage to a more general usage in many languages, including English, German, French, Italian, Spanish, etc. In English, it is used in fields such as economics, philosophy, and politics, with particular meanings in medicine and law.

Andrew Jackson, an early American president, gave a toast 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".

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