It looks like everyone has it's own theories on what's the function of a defense. People say that it has to be a contribution to knowledge, other people say that it has to reflect 3 years of work, other say that it has to be a work of publishable quality. Given that "contribution to knowledge" is kinda a difficult ground to judge for some humanities, how are examiners approaching a PhD defense in the humanities?
Because there are so many different disciplines and educational systems, there is no universal definition of what constitutes either sufficient work for a PhD or constitutes a successful defense.
That said, the “original work of scholarship” connotes that the PhD candidate has produced something that is new at the time of the defense. In the arts, for instance, is an exhibition of sculptures or paintings “publishable?” That depends greatly on your definition, but it would unquestionably represent something original.
The goal of the defense is also quite variable, depending on local policies and culture but is usually regarded as a vehicle for demonstrating mastery to an audience by allowing for questioning of the candidate.
The ‘defense’ is part and parcel of the work itself. Typically you must first produce some novel work, and at the very end, you must ‘defend’ this work in an act of oral discourse. The defense is mostly a rite of passage in most disciplines. Your advisor and committee usually won’t let you get that far if the work is sub par and you’re likely to ‘fail’. Officially, you must be able to speak intelligently about the work and address criticisms on the fly, proving your scholarship and deep knowledge.