An alternate title could be "Do we award a PhD for what the student became or for what he produced?". More precisely, if a research thesis report does not advance human knowledge, yet still shows that the student acquired the right skills of investigation, should the title of Philosophy Doctor be awarded?
I kind of always assumed that a thesis (PhD thesis in particular) should have some positive results (where positive means "advancing the state of human knowledge"), and that part of the art of finding a good topic of research was the art of asking the right kind of question, which would yield some positive result independently of the answer. Similar views are expressed in this other stackexchange question and the corresponding answers:
It is an expectation that the PhD would make an original contribution and/or advance knowledge in a given field. I understand this is a universal assumption for this level of study across all universities. (...) usually a PhD is measured on its contribution to expand knowledge.
Nevertheless, Justin Zobel defends convincingly the opposite view in "Writing for Computer Science", p.154 of the Second Edition:
even if good results are not achieved, the thesis should pass if you have shown the ability to undertake high-quality research. (...) A thesis with negative results can, if appropriately written, demonstrate the ability of the candidate just as well a a thesis with positive results. (...) it is you, not the research, that is the primary object of scrutiny
Is there an agreement across disciplines about this question?
I am not directly concerned (I reported positive results in my PhD thesis long ago, have many positive results to report in my "Habilitation" thesis, and I certainly aim for my students to report positive results in theirs), but I am curious about the real objective of the thesis: - as an advisor, I could suggest a more risky topic if it had the potential to teach more to the student without risking the whole graduating thing; and - as a referee or member of an evaluation committee, I have to judge students and/or their thesis...
Extreme Fictional Example
A student and advisor do the entire research work following the most rigorous scientific process for several years, only to find their efforts ruined near the end of the process either by a budget cut, the disappearance of the species they were studying, or the discovery that the problem is the consequence of an obscure results from year ago in another research community.
The student has followed and learned the scientific process, but did not contribute to human knowledge (apart from maybe improving the index of its bibliography). If the student has showed the qualities required from a good researcher, should(n't) he/she be awarded the title of "Doctor in Philosophy", independently of the contribution made to human knowledge?
This is truly a rhetorical question, and I doubt this kind of situation happens often. Yet the idea is new to me and I kind of like it, albeit I doubt the whole community would agree...
Opposite Extreme Example
Imagine that a student, stroke by luck, makes an amazing scientific discovery which deeply impact human knowledge, and can be understood by all even though the student poorly redacts it. It seems clear to me that the society would not benefit from awarding a PhD to such a student, who has not learned how to do research even though contributing to human knowledge.
On the other hand, setting two conditions for the awarding of a PhD, having learned how to do proper research AND having advanced human knowledge by using it, introduces trade-offs and compromises (which again do not serve society).