What is a PhD?
A PhD is both a process and a qualification.
As a process, it exists to help you develop the skills of a professional academic researcher.
As a qualification, it shows that you've developed the skills required to do professional academic research.
What do you have to achieve?
A PhD doesn't have to be "a significant contribution to the body of knowledge". Most research published by professional academics has very little impact, so this can't be the minimum standard we demand from students.
Nor do you have to be "the world's leading expert in your field". If every PhD graduate is the leading expert in their field, each is the leading expert in a field of one.
A PhD is not the end of the journey nor the culmination of your life's work. And it probably won't be the work you are remembered for. It's the first step of an academic career and you potentially have decades to continue to learn and make an impact.
Instead, you need to gain, then show, competence;
- You need to develop a good foundational grasp of the literature in your niche
- You need to find a problem that's worth working on and use appropriate methods and tools to investigate it
- You need the skill to carry out your methods competently
- You need to be able to understand the results your methods give you and adapt if necessary
- You need to be able to present your work and you need to be able to discuss your work with other academics.
How do you develop these skills?
Crucially, this is not a linear process. You don't just read literature, then find a problem, then do the research, analyse it and present it.
It is a cyclical process. You should go through these steps over and over again, going back to the literature, practicing your research methods, analysing the results, discussing your work, adapting, going back to the literature... Your practical experience will make it easier to understand the literature, which you can then reapply to the practical work.
The cycle of feedback (whether your own assessment or someone else's) and adaptation is essential to develop any skill. And the more often you go through the cycle, the better.
If you were learning to be a chef, you wouldn't spend 2 years reading cookbooks (or writing a review to summarize them), then plan a 10-course meal for 30 people and expect to get a Michelin star. You'd read a bit, practice, taste, get feedback and advice from others, adapt and go again (and again and again). You'd learn by making mistakes and adapting and gradually adding more difficulty (in terms of technique and/or scale).
How do you know when you have enough?
While people may define the standard in different (and often subjective) ways, the basic aim of a PhD is the same as that of any professional academic research; to produce work of a publishable standard.
This will partly depend on what's already been done around the problem you're working on. It isn't enough, though, to just be original. You need to reach a level of execution comparable to the standards typically seen in your field. You don't have to be the world's best, just professionally competent.
Though there are different requirements in different countries, as a general rule if you have 2 or 3 publications (or work that could be published), it's a good demonstration that you're able to reach that professional academic standard.