Some things are important and some things are absolutely necessary. You can go for many days without food, but without water, your days are really numbered. Water is the sine qua non of human life.
Sine qua non means literally “without which is nothing”. It describes an essential condition or requirement without which something cannot exist. Due process is the sine qua non of justice. The element hydrogen is the sine qua non of the universe as we know it.
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BBC&B