sine qua non

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.