...whereas some degree of political freedom is a sine qua non condition for contestation, democracy cannot be sufficiently defined in terms of liberties....
Latin phrase which means "without which"--refers to something which is needed in order to make the program work at all, i e without this item, a program will not work
A sine qua non is something that is essential if you want to achieve a particular thing. Successful agricultural reform is also a sine qua non of Mexico's modernisation. something that you must have, or which must exist, before something else can happen sine qua non for/of