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'''Sine qua non''' or '''conditio sine qua non''' was originally a Latin legal term for "without which it could not be" ("but for"). It refers to an indispensable and essential action, condition, or ingredient. 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 Andrew Jackson on the occasion of Jackson receiving an honorary doctorate from Harvard. The President responded to his listeners, "E pluribus unum, my friends. Sine qua non."