Sine qua non

Near-synonyms: dealbreaker, necessary condition

it is hardly likely he would have found any one so singularly interesting and fascinating as Margarita Riccardini; for the striking and animated beauty of her father was softened and relieved by that peculiar something, half modesty, and half pride, which is the characteristic of English loveliness, and which every Englishman requires as a sine qua non ere he resigns himself to a bondage it is the habit of his nature, or the result of his privileges, to admit reluctantly.

Beauty, grace, and accomplishments are, as I said before, a sine qua non, but as Basil is already the colonel of a crack cavalry corps, and has other military honours promised him in the near future, as also by reason that his father is Governor-General of the gubernium of Odessa, and will leave his son abundant wealth as well as his title of count, I think I have a right to expect something more in his future wife than mere accomplishments and good looks.

Literature, instead of being an accessory, is the fundamental sine qua non of complete living.

[…] whereas some degree of political freedom is a sine qua non condition for contestation, democracy cannot be sufficiently defined in terms of “liberties”[…]