When I arrived to this question, there was (and probably still is) a highly upvoted comment:
"I fulfilled all of the requirements." — Austin Henley
I think that it's very easy for a new PhD or candidate to have serious doubts about the quality of their work, and to suffer a bit of the impostor syndrome. After all, they've spent numerous years seeing just how much knowledge is out there, and realizing that despite their new expertise, in the big scheme of things, it's actually a pretty small piece.
I've just completed my PhD, and one of the things that my advisor mentioned to me was very helpful, especially when I was feeling a bit of what I've described above. At my university, PhD students become candidates when they complete their candidacy, which includes their candidacy proposal, in which the student presents their research proposal to a committee (typically with members of their eventual examining committee) who must approve the proposal. My advisor reminded me that regardless of my own perspective on my research, the fact of the matter is that three or four years ago, a committee of experienced researchers, professors, etc., (i.e., my candidacy committee) reviewed my proposal and confirmed that the work it describes merits a PhD.
Based on that reminder, I framed by defense slides by beginning with a very quick review of the original candidacy, including a slide with a short problem statement. At the end of the defense, I pulled up another copy of that slide and addressed each point in the original problem statement, explaining how I'd addressed it. Then I followed with some "reveal text":
quod erat demonstrandum
My research involved a fair amount of proof theory and formal logic, so this was especially appropriate, but the point remains: the work that I presented was that "which had to be demonstrated". The experienced committee said several years ago that the proposed work merits a PhD, and I completed that work.
The point here is that it's not really the candidate's place to determine what merits a PhD. They haven't, at that time, enough experience to make that determination. The university and committee does, and has already decided what merits a PhD, and it would be entirely appropriate to respond to the question "What merits a PhD?" with "you, as a committee member, explained that to me some number of years ago, and I've fulfilled those requirements."